feat(launch): isolate harness homes and record immutable launch provenance

Mosaic wrote into the operator's harness base installs — ~/.claude,
~/.pi/agent, ~/.codex, ~/.config/opencode — for settings, instructions, and a
102-symlink skill farm per harness. Any experiment with hooks or gating
therefore mutated the operator's own tooling, and a broken framework change
could take out the very harness needed to repair it.

Harness home isolation
----------------------
Each runtime now reads config from a dedicated mosaic-owned home via the
harness's own config-dir variable:

  claude    CLAUDE_CONFIG_DIR     ~/.config/mosaic/.claude
  pi        PI_CODING_AGENT_DIR   ~/.config/mosaic/.pi     (replaces ~/.pi/agent)
  codex     CODEX_HOME            ~/.config/mosaic/.codex
  opencode  XDG_CONFIG_HOME       ~/.config/mosaic/.opencode

These paths are manifest-UNKNOWN, so rule 3 (#791) resolves them to operator
ownership and a keep-mode upgrade can neither overwrite nor prune them.
A bare `claude` / `pi` keeps its own config AND auth, making it a structural
break-glass rather than one depending on restoring a file under pressure.

opencode is blunter than the rest: it has no dedicated variable and follows XDG,
so isolation also relocates XDG lookups for anything it spawns. Documented in
place.

mosaic-sync-skills now links into those homes and cleans the legacy farms it
previously planted in base installs. Ownership is proven by RESOLUTION, not by
name — only symlinks resolving inside the canonical/local skills dirs are
removed, mirroring the refusal already in commands/skill.js. Verified against a
real install: codex's own .system directory survived while its 102 mosaic links
were removed. Both resolution prefixes are length-checked first; an empty prefix
would make "$resolved" == "$prefix/"* match every absolute path and delete
foreign symlinks.

Immutable launch record
-----------------------
Every launch now appends one record to fleet/run/sessions/events.ndjson before
exec. Mandatory, mechanical, no model involvement.

pi rewrites its own argv to a bare `pi`, so /proc/<pid>/cmdline destroys the
launch evidence — that has already produced a confident wrong diagnosis ("this
agent bypassed the launcher"), disproved only by the parent's argv and only
because the parent had not yet exited. A record written before exec is the only
place this survives.

The path is the #797 Runtime Session Ledger, already operator-classified and
already covered by test-upgrade-manifest-guard.sh, which seeds it and proves a
populated ledger survives keep-mode upgrades — but nothing shipped ever wrote
it. This implements it in the shape that guard already asserts (0600 files under
a 0700 dir).

`mosaic` writes session.launch; launch-runtime.py appends lease.register with
the broker session id and activation capability. They correlate by an explicit
MOSAIC_LAUNCH_ID, never by pid: execRuntime uses spawnSync, so the runtime is a
child with a different pid.

Records normative fragment digests (CONSTITUTION/AGENTS/SOUL/USER/STANDARDS/
TOOLS/RUNTIME) — the same set the broker hashes for promotion, so drift is
mechanically detectable rather than a matter of judgement.

Credential-safe: env is captured as PRESENT NAMES ONLY, and argv values over
256 bytes become a sha256 + length rather than being inlined.

Also fixes CLI_VERSION resolution: '@mosaicstack/mosaic/package.json' is not in
the package exports map and always throws ERR_PACKAGE_PATH_NOT_EXPORTED.
resolveTool() uses that same failing specifier, which is why its documented
preference for bundled tools over the deployed ~/.config/mosaic copy has never
once applied — noted in place, not fixed here.

Verified on sb-it-1-dt: isolated homes written and base installs byte-identical
for all four harnesses; 408 legacy symlinks removed with 1 foreign entry
preserved; launch records paired across the spawn boundary. typecheck shows zero
errors in launch.ts (the @mosaicstack/types failures are pre-existing and
reproduce on a pristine origin/main worktree).

packages/mosaic/framework/tools/_scripts/mosaic-sync-skills

@@ -153,7 +153,24 @@ if [[ $link_only -eq 1 ]]; then
   exit 0
 fi
 
+# Skills are linked into the MOSAIC-OWNED harness homes, never a base install.
+# Paths mirror the config-dir env vars the launcher injects (HARNESS_HOME_ENV in
+# commands/launch.js):
+#   claude   CLAUDE_CONFIG_DIR      -> <home>/skills
+#   pi       PI_CODING_AGENT_DIR    -> <home>/skills   (replaces ~/.pi/agent)
+#   codex    CODEX_HOME             -> <home>/skills
+#   opencode XDG_CONFIG_HOME        -> <home>/opencode/skills  (XDG adds a level)
 link_targets=(
+  "$MOSAIC_HOME/.claude/skills"
+  "$MOSAIC_HOME/.codex/skills"
+  "$MOSAIC_HOME/.opencode/opencode/skills"
+  "$MOSAIC_HOME/.pi/skills"
+)
+
+# Pre-isolation installs planted the same symlink farm directly in the operator's
+# base installs. Those are now orphaned: the launcher no longer reads them, but
+# they persist and make a "clean" base install look mosaic-managed.
+legacy_link_targets=(
   "$HOME/.claude/skills"
   "$HOME/.codex/skills"
   "$HOME/.config/opencode/skills"
@@ -252,6 +269,60 @@ prune_stale_links_in_target() {
   done < <(find "$target_dir" -mindepth 1 -maxdepth 1 -type l -print0)
 }
 
+# Remove mosaic-owned symlinks left in a base install by a pre-isolation sync.
+#
+# Ownership is proven by RESOLUTION, not by name: only links resolving inside the
+# canonical or local skills dirs are removed. Anything else — a real directory, a
+# link elsewhere, an unresolvable link — is left untouched. This mirrors the
+# refusal in commands/skill.js ("only symlinks pointing inside the Mosaic skills
+# directory are managed") and preserves e.g. codex's own `.system` dir.
+#
+# The directory itself is kept: mosaic-doctor warns when ~/.pi/agent/skills is
+# missing, and an empty dir is the correct end state, not an absent one.
+cleanup_legacy_target() {
+  local target_dir="$1"
+  local removed=0 kept=0
+
+  [[ -d "$target_dir" ]] || return 0
+
+  while IFS= read -r -d '' link_path; do
+    local resolved owned=0
+    resolved="$(readlink -m "$link_path" 2>/dev/null || true)"
+
+    # Guard the empty-prefix trap: an unset *_real would make "$resolved" == "/"*
+    # match every absolute path and delete foreign links.
+    if [[ -n "$resolved" ]]; then
+      if [[ -n "$canonical_real" && "$resolved" == "$canonical_real/"* ]]; then
+        owned=1
+      elif [[ -n "$local_real" && "$resolved" == "$local_real/"* ]]; then
+        owned=1
+      fi
+    fi
+
+    if [[ $owned -eq 1 ]]; then
+      rm -f "$link_path"
+      removed=$((removed + 1))
+    else
+      kept=$((kept + 1))
+    fi
+  done < <(find "$target_dir" -mindepth 1 -maxdepth 1 -type l -print0)
+
+  if [[ $removed -gt 0 ]]; then
+    echo "[mosaic-skills] Legacy cleanup: removed $removed mosaic symlink(s) from $target_dir (preserved $kept foreign)"
+  fi
+}
+
+for legacy in "${legacy_link_targets[@]}"; do
+  # Skip anything that is also a current target, so isolation can never
+  # self-destruct if the two lists ever overlap.
+  skip=0
+  for target in "${link_targets[@]}"; do
+    [[ "$legacy" == "$target" ]] && skip=1
+  done
+  [[ $skip -eq 1 ]] && continue
+  cleanup_legacy_target "$legacy"
+done
+
 for target in "${link_targets[@]}"; do
   mkdir -p "$target"
 

packages/mosaic/framework/tools/lease-broker/launch-runtime.py

@@ -8,7 +8,9 @@ import json
 import os
 import socket
 import sys
+import time
 from collections.abc import Callable, Mapping, Sequence
+from datetime import datetime, timezone
 from pathlib import Path
 from typing import Final
 
@@ -53,6 +55,48 @@ def broker_request(socket_path: Path, request: dict[str, object]) -> dict[str, o
     return value
 
 
+def _self_starttime() -> str | None:
+    """Field 22 of our own /proc stat — the anchor starttime the broker records.
+
+    Read past the comm field's parens, since a process name may contain them.
+    """
+    try:
+        raw = Path(f"/proc/{os.getpid()}/stat").read_text()
+        return raw.rsplit(")", 1)[1].split()[19]
+    except (OSError, IndexError, ValueError):
+        return None
+
+
+def _append_launch_record(environ: Mapping[str, str], record: dict[str, object]) -> None:
+    """Append one NDJSON event to the #797 Runtime Session Ledger.
+
+    `fleet/run/sessions/` is operator-classified in framework-manifest.txt and is
+    already covered by test-upgrade-manifest-guard.sh, so an upgrade can neither
+    overwrite nor prune it. Files 0600 under a 0700 dir, matching what that guard
+    asserts.
+
+    Never raises: a launch must not be denied over bookkeeping. But it also never
+    fails silently — a missing record is exactly the kind of gap that made the
+    2026-08-06 MUTATOR_UNVERIFIED investigation cost a day.
+    """
+    try:
+        mosaic_home = environ.get("MOSAIC_HOME") or str(Path.home() / ".config" / "mosaic")
+        directory = Path(mosaic_home) / "fleet" / "run" / "sessions"
+        directory.mkdir(parents=True, exist_ok=True)
+        os.chmod(directory, 0o700)
+        framed = {
+            "seq": time.time_ns() // 1_000_000,
+            "ts": datetime.now(timezone.utc).isoformat(),
+            **record,
+        }
+        path = directory / "events.ndjson"
+        descriptor = os.open(path, os.O_WRONLY | os.O_CREAT | os.O_APPEND, 0o600)
+        with os.fdopen(descriptor, "w") as handle:
+            handle.write(json.dumps(framed, separators=(",", ":")) + "\n")
+    except (OSError, ValueError, TypeError) as error:
+        print(f"[mosaic] WARNING: launch record not written: {error}", file=sys.stderr)
+
+
 def main(
     argv: Sequence[str] | None = None,
     *,
@@ -94,8 +138,9 @@ def main(
     # silent pass and never folded into the generic registration-failure
     # branch.
     try:
+        activation_capability = probe_activation_capability(source_environment)
         assert_activation_capability_matches(
-            probe_activation_capability(source_environment),
+            activation_capability,
             expected_activation_capability,
         )
     except VersionCouplingError as version_error:
@@ -128,6 +173,32 @@ def main(
         print("Mosaic lease broker registration failed; runtime launch denied.", file=sys.stderr)
         return 1
 
+    # Immutable launch record, half two. `mosaic` wrote `session.launch` with the
+    # config/provenance it knows; only this process knows the broker session id
+    # and the activation capability it just asserted. os.execvpe preserves the
+    # PID, so this PID is BOTH the anchor pid and the join key back to that
+    # record. Never fatal — bookkeeping must not deny a launch — but never
+    # silent either.
+    _append_launch_record(
+        source_environment,
+        {
+            "kind": "lease.register",
+            # Joins back to `mosaic`'s session.launch record. NOT pid: execRuntime()
+            # spawns rather than execs, so this process is a CHILD of mosaic with a
+            # different pid. This pid IS the broker anchor pid (os.execvpe below
+            # preserves it), which is a separate and still-useful fact.
+            "launch_id": source_environment.get("MOSAIC_LAUNCH_ID"),
+            "pid": os.getpid(),
+            "runtime": arguments.runtime,
+            "session_id": session_id,
+            "runtime_generation": generation,
+            "generation_file": str(generation_file),
+            "anchor_starttime": _self_starttime(),
+            "activation_capability": activation_capability,
+            "command": Path(command[0]).name,
+        },
+    )
+
     environment = dict(source_environment)
     environment["MOSAIC_LEASE_SESSION_ID"] = session_id
     environment["MOSAIC_RUNTIME_GENERATION"] = str(generation)

packages/mosaic/src/commands/launch.ts

@@ -14,9 +14,11 @@ import {
   readdirSync,
   realpathSync,
   rmSync,
+  appendFileSync,
 } from 'node:fs';
+import { createHash, randomBytes } from 'node:crypto';
 import { createRequire } from 'node:module';
-import { homedir } from 'node:os';
+import { homedir, hostname } from 'node:os';
 import { join, dirname } from 'node:path';
 import type { Command } from 'commander';
 import {
@@ -42,6 +44,163 @@ const RUNTIME_LABELS: Record<RuntimeName, string> = {
   pi: 'Pi',
 };
 
+// ─── Harness home isolation ──────────────────────────────────────────────────
+// Mosaic-launched runtimes read config from a dedicated home under the mosaic
+// tree — never the operator's base install. A bare `claude` / `pi` therefore
+// keeps its own config AND its own auth, and stays a working break-glass no
+// matter what mosaic does to its own tree.
+//
+// These paths are manifest-UNKNOWN, which resolves to operator ownership
+// (framework-manifest.txt rule 3, #791), so a keep-mode `mosaic update` can
+// neither overwrite nor prune them. Overwrite-mode install still would.
+//
+// opencode has no dedicated config-dir variable and follows XDG, so isolating it
+// sets XDG_CONFIG_HOME for that process tree. That is blunter than the other
+// three: it also relocates XDG lookups for anything opencode spawns.
+const HARNESS_HOME_ENV: Record<RuntimeName, string> = {
+  claude: 'CLAUDE_CONFIG_DIR',
+  pi: 'PI_CODING_AGENT_DIR',
+  codex: 'CODEX_HOME',
+  opencode: 'XDG_CONFIG_HOME',
+};
+
+/** Dedicated mosaic-owned home for a runtime: ~/.config/mosaic/.<runtime> */
+function harnessHome(runtime: RuntimeName): string {
+  return join(MOSAIC_HOME, `.${runtime}`);
+}
+
+/**
+ * Env overlay pointing a runtime at its mosaic-owned home. The directory is
+ * created on demand so a first launch does not fail on a missing path.
+ */
+function harnessEnv(runtime: RuntimeName): Record<string, string> {
+  const key = HARNESS_HOME_ENV[runtime];
+  if (!key) return {};
+  const home = harnessHome(runtime);
+  mkdirSync(home, { recursive: true });
+  return { [key]: home };
+}
+
+// ─── Launch record (immutable provenance) ────────────────────────────────────
+// MANDATORY and MECHANICAL: every launch appends one record of what the agent
+// actually launched with, written before exec. No model involvement, no opt-out.
+//
+// WHY LAUNCH-TIME AND NOT INSPECT-LATER: pi rewrites its own argv to a bare
+// `pi`, so /proc/<pid>/cmdline DESTROYS the launch evidence. That has already
+// produced a confident wrong diagnosis ("this agent bypassed the launcher"),
+// disproved only by the parent process's argv and only because the parent had
+// not yet exited. A record written before exec is the only place this survives.
+//
+// Lands in fleet/run/sessions/ — the #797 Runtime Session Ledger path, already
+// operator-classified in framework-manifest.txt and already covered by
+// test-upgrade-manifest-guard.sh, so an upgrade can neither overwrite nor prune
+// it.
+//
+// CORRELATION is by an explicit MOSAIC_LAUNCH_ID, never by pid: execRuntime()
+// uses spawnSync, so the runtime is a CHILD with a different pid.
+// launch-runtime.py appends the matching `lease.register` event.
+//
+// NEVER records a credential value: env is captured as PRESENT NAMES ONLY, and
+// oversized argv values (the composed system prompt) become a digest + length.
+const LAUNCH_LEDGER_DIR = join(MOSAIC_HOME, 'fleet', 'run', 'sessions');
+
+const CLI_VERSION: string | null = (() => {
+  try {
+    // Resolved RELATIVELY: the package `exports` map does not expose
+    // package.json, so '@mosaicstack/mosaic/package.json' throws
+    // ERR_PACKAGE_PATH_NOT_EXPORTED. Same relative depth from src/ and dist/.
+    return (createRequire(import.meta.url)('../../package.json') as { version: string }).version;
+  } catch {
+    return null;
+  }
+})();
+
+interface NormativeFragmentDigest {
+  source_id: string;
+  sha256: string | null;
+  bytes: number | null;
+  missing?: boolean;
+}
+
+function sha256Of(value: string | Buffer): string {
+  return createHash('sha256').update(value).digest('hex');
+}
+
+/**
+ * Hash the normative sources injected into the agent. This is "what the agent
+ * IS" — and it is the same fragment set the lease broker hashes for promotion,
+ * so an unexpected digest here is a mechanically detectable red flag rather than
+ * a matter of judgement.
+ */
+function normativeFragmentDigests(runtime: RuntimeName): NormativeFragmentDigest[] {
+  const candidates: Array<[string, string]> = [
+    ['CONSTITUTION.md', join(MOSAIC_HOME, 'CONSTITUTION.md')],
+    ['AGENTS.md', join(MOSAIC_HOME, 'AGENTS.md')],
+    ['SOUL.md', join(MOSAIC_HOME, 'SOUL.md')],
+    ['USER.md', join(MOSAIC_HOME, 'USER.md')],
+    ['STANDARDS.md', join(MOSAIC_HOME, 'STANDARDS.md')],
+    ['TOOLS.md', join(MOSAIC_HOME, 'TOOLS.md')],
+    [`runtime/${runtime}/RUNTIME.md`, join(MOSAIC_HOME, 'runtime', runtime, 'RUNTIME.md')],
+  ];
+  return candidates.map(([sourceId, path]) => {
+    try {
+      const bytes = readFileSync(path);
+      return { source_id: sourceId, sha256: sha256Of(bytes), bytes: bytes.length };
+    } catch {
+      return { source_id: sourceId, sha256: null, bytes: null, missing: true };
+    }
+  });
+}
+
+/** argv with oversized values replaced by a digest, so the record stays small
+ * and never inlines injected content verbatim. */
+function redactArgv(argv: string[]): string[] {
+  return argv.map((a) =>
+    typeof a === 'string' && a.length > 256
+      ? `<redacted sha256:${sha256Of(a).slice(0, 16)} bytes:${a.length}>`
+      : a,
+  );
+}
+
+function recordLaunch(runtime: RuntimeName, cliArgs: string[], yolo: boolean): void {
+  try {
+    mkdirSync(LAUNCH_LEDGER_DIR, { recursive: true, mode: 0o700 });
+    // Correlation id for the lease.register half. Set into process.env so it
+    // propagates through every `...process.env` / `...baseEnv` spread below.
+    const launchId = `${Date.now().toString(36)}-${randomBytes(6).toString('hex')}`;
+    process.env['MOSAIC_LAUNCH_ID'] = launchId;
+    const record = {
+      seq: Date.now(),
+      kind: 'session.launch',
+      launch_id: launchId,
+      ts: new Date().toISOString(),
+      host: hostname(),
+      pid: process.pid,
+      runtime,
+      mode: yolo ? 'yolo' : 'normal',
+      cwd: process.cwd(),
+      cli_version: CLI_VERSION,
+      config_home: harnessHome(runtime),
+      config_home_isolated: true,
+      config_home_env: HARNESS_HOME_ENV[runtime] ?? null,
+      argv: redactArgv(cliArgs),
+      normative_fragments: normativeFragmentDigests(runtime),
+      // names only — values are never recorded
+      mosaic_env_present: Object.keys(process.env)
+        .filter((k) => k.startsWith('MOSAIC_'))
+        .sort(),
+    };
+    appendFileSync(join(LAUNCH_LEDGER_DIR, 'events.ndjson'), `${JSON.stringify(record)}\n`, {
+      mode: 0o600,
+    });
+  } catch (err) {
+    // Never block a launch on bookkeeping — but never fail silently either.
+    console.error(
+      `[mosaic] WARNING: launch record not written: ${err instanceof Error ? err.message : String(err)}`,
+    );
+  }
+}
+
 // ─── Pre-flight checks ──────────────────────────────────────────────────────
 
 function checkMosaicHome(): void {
@@ -105,11 +264,11 @@ interface SettingsAudit {
 
 function auditClaudeSettings(): SettingsAudit {
   const warnings: string[] = [];
-  const settingsPath = join(homedir(), '.claude', 'settings.json');
+  const settingsPath = join(harnessHome('claude'), 'settings.json');
   const settings = readJson(settingsPath);
 
   if (!settings) {
-    warnings.push('~/.claude/settings.json not found — hooks and plugins will be missing');
+    warnings.push(`${settingsPath} not found — hooks and plugins will be missing`);
     return { warnings };
   }
 
@@ -561,7 +720,9 @@ function skillRealPath(dir: string): string {
 /** Skill roots Pi auto-discovers natively (no `--skill` needed): its global
  * skills dir and the project-local one relative to the launch cwd. */
 function piNativeSkillRoots(cwd: string = process.cwd()): string[] {
-  return [join(homedir(), '.pi', 'agent', 'skills'), join(cwd, '.pi', 'skills')];
+  // PI_CODING_AGENT_DIR replaces ~/.pi/agent (not ~/.pi), so skills live at
+  // <home>/skills — there is no extra 'agent' segment under the isolated home.
+  return [join(harnessHome('pi'), 'skills'), join(cwd, '.pi', 'skills')];
 }
 
 /** Enumerate skill dirs under a set of roots, deduped by real path. A directory
@@ -764,12 +925,13 @@ function launchRuntime(runtime: RuntimeName, args: string[], yolo: boolean): nev
         cliArgs.push(...args);
       }
       console.log(`[mosaic] Launching ${label}${modeStr}${missionStr}...`);
+      recordLaunch('claude', cliArgs, yolo);
       execLeaseGatedRuntime('claude', cliArgs, process.env, yolo);
       break;
     }
 
     case 'codex': {
-      ensureRuntimeConfig('codex', join(homedir(), '.codex', 'instructions.md'));
+      ensureRuntimeConfig('codex', join(harnessHome('codex'), 'instructions.md'));
       const cliArgs = yolo ? ['--dangerously-bypass-approvals-and-sandbox'] : [];
       if (hasMissionNoArgs) {
         cliArgs.push(missionPrompt);
@@ -777,14 +939,17 @@ function launchRuntime(runtime: RuntimeName, args: string[], yolo: boolean): nev
         cliArgs.push(...args);
       }
       console.log(`[mosaic] Launching ${label}${modeStr}${missionStr}...`);
-      execRuntime('codex', cliArgs);
+      recordLaunch('codex', cliArgs, yolo);
+      execRuntime('codex', cliArgs, { ...process.env, ...harnessEnv('codex') });
       break;
     }
 
     case 'opencode': {
-      ensureRuntimeConfig('opencode', join(homedir(), '.config', 'opencode', 'AGENTS.md'));
+      // opencode follows XDG, so its config resolves to $XDG_CONFIG_HOME/opencode.
+      ensureRuntimeConfig('opencode', join(harnessHome('opencode'), 'opencode', 'AGENTS.md'));
       console.log(`[mosaic] Launching ${label}${modeStr}...`);
-      execRuntime('opencode', args);
+      recordLaunch('opencode', args, yolo);
+      execRuntime('opencode', args, { ...process.env, ...harnessEnv('opencode') });
       break;
     }
 
@@ -799,6 +964,7 @@ function launchRuntime(runtime: RuntimeName, args: string[], yolo: boolean): nev
         cliArgs.push(...args);
       }
       console.log(`[mosaic] Launching ${label}${modeStr}${missionStr}...`);
+      recordLaunch('pi', cliArgs, yolo);
       execLeaseGatedRuntime('pi', cliArgs);
       break;
     }
@@ -835,6 +1001,7 @@ function execLeaseGatedRuntime(
     [launcher, ...dangerousArgs, '--runtime', runtime, '--', runtime, ...args],
     {
       ...baseEnv,
+      ...harnessEnv(runtime),
       MOSAIC_LEASE_BROKER_SOCKET: defaultLeaseBrokerSocket(baseEnv),
       MOSAIC_RUNTIME_GENERATION: baseEnv['MOSAIC_RUNTIME_GENERATION'] ?? '1',
     },