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"