feat(launch): per-agent harness homes + seat persona layer (MOSAIC-D-002)

With an active brain seat (MOSAIC_AGENT_NAME + seat dir under the brain
home), 'mosaic launch' now gives the runtime a per-agent home inside the
seat dir — <brainHome>/fleet/agents/<seat>/.<runtime> — instead of the
shared per-runtime ~/.config/mosaic/.<runtime>. Per-agent sessions,
settings, and auth live inside the seat (canon §2; dot-named so brain
ignore policy keeps credential material untracked).

- activeSeatDir(): resolves the seat only when a brain is active; agent
  names validated against a safe charset (path traversal rejected)
- seatPersonaOverlay(): <seat>/SOUL.md injected by value as a '## Seat
  Persona' overlay block layering persona on the root generic base
- launch record: config_home_kind (seat|runtime-shared) + agent_name
- bare launches (no MOSAIC_AGENT_NAME) and non-seat agents keep the
  shared home — no behavior change without a brain

Tests: launch.spec seat-home cases incl. unsafe-name matrix + persona
overlay hermetic tests (no host-roster dependency).

packages/mosaic/src/commands/launch.spec.ts

@@ -349,3 +349,90 @@ describe('registerRuntimeLaunchers — claudex (EXPERIMENTAL overlay)', () => {
     expect(mockExit).not.toHaveBeenCalled();
   });
 });
+
+// ─── Seat harness homes (MOSAIC-D-002, brain-home split) ────────────────────
+
+import { activeSeatDir, seatPersonaOverlay } from './launch.js';
+
+describe('activeSeatDir — per-agent harness home resolution', () => {
+  let root: string;
+  const savedAgentName = process.env['MOSAIC_AGENT_NAME'];
+  const savedBrainHome = process.env['MOSAIC_BRAIN_HOME'];
+
+  beforeEach(() => {
+    root = mkdtempSync(join(tmpdir(), 'mosaic-seat-home-'));
+    delete process.env['MOSAIC_BRAIN_HOME'];
+  });
+
+  afterEach(() => {
+    rmSync(root, { recursive: true, force: true });
+    if (savedAgentName === undefined) {
+      delete process.env['MOSAIC_AGENT_NAME'];
+    } else {
+      process.env['MOSAIC_AGENT_NAME'] = savedAgentName;
+    }
+    if (savedBrainHome !== undefined) {
+      process.env['MOSAIC_BRAIN_HOME'] = savedBrainHome;
+    } else {
+      delete process.env['MOSAIC_BRAIN_HOME'];
+    }
+  });
+
+  it('resolves the seat dir when MOSAIC_BRAIN_HOME carries the seat', () => {
+    const seat = join(root, 'brain', 'fleet', 'agents', 'coder0');
+    mkdirSync(seat, { recursive: true });
+    process.env['MOSAIC_AGENT_NAME'] = 'coder0';
+    process.env['MOSAIC_BRAIN_HOME'] = join(root, 'brain');
+
+    expect(activeSeatDir(join(root, 'config', 'mosaic'))).toBe(seat);
+  });
+
+  it('returns undefined without an agent name (bare launches stay shared)', () => {
+    delete process.env['MOSAIC_AGENT_NAME'];
+    expect(activeSeatDir(join(root, 'config', 'mosaic'))).toBeUndefined();
+  });
+
+  it('returns undefined when the seat dir does not exist in the brain', () => {
+    process.env['MOSAIC_AGENT_NAME'] = 'ghost';
+    process.env['MOSAIC_BRAIN_HOME'] = join(root, 'brain');
+    mkdirSync(join(root, 'brain', 'fleet', 'agents'), { recursive: true });
+
+    expect(activeSeatDir(join(root, 'config', 'mosaic'))).toBeUndefined();
+  });
+
+  it.each(['../escape', 'a/b', '.hidden-start', '', 'spaced name'])(
+    'rejects unsafe agent name %j (path traversal cannot leave the seat store)',
+    (name: string) => {
+      process.env['MOSAIC_AGENT_NAME'] = name;
+      process.env['MOSAIC_BRAIN_HOME'] = join(root, 'brain');
+
+      expect(activeSeatDir(join(root, 'config', 'mosaic'))).toBeUndefined();
+    },
+  );
+
+  it('seatPersonaOverlay renders the seat SOUL.md as an overlay block', () => {
+    const seat = join(root, 'brain', 'fleet', 'agents', 'coder0');
+    mkdirSync(seat, { recursive: true });
+    writeFileSync(join(seat, 'SOUL.md'), '# coder0 — code seat persona\n\nShips tested code.\n');
+    process.env['MOSAIC_AGENT_NAME'] = 'coder0';
+    process.env['MOSAIC_BRAIN_HOME'] = join(root, 'brain');
+
+    const overlay = seatPersonaOverlay(join(root, 'config', 'mosaic'));
+    expect(overlay).toContain('## Seat Persona');
+    expect(overlay).toContain('coder0 — code seat persona');
+  });
+
+  it('seatPersonaOverlay is empty when the seat carries no SOUL.md', () => {
+    const seat = join(root, 'brain', 'fleet', 'agents', 'coder0');
+    mkdirSync(seat, { recursive: true });
+    process.env['MOSAIC_AGENT_NAME'] = 'coder0';
+    process.env['MOSAIC_BRAIN_HOME'] = join(root, 'brain');
+
+    expect(seatPersonaOverlay(join(root, 'config', 'mosaic'))).toBe('');
+  });
+
+  it('seatPersonaOverlay is empty when no agent name is set', () => {
+    delete process.env['MOSAIC_AGENT_NAME'];
+    expect(seatPersonaOverlay(join(root, 'config', 'mosaic'))).toBe('');
+  });
+});

packages/mosaic/src/commands/launch.ts

@@ -19,7 +19,7 @@ import {
 import { createHash, randomBytes } from 'node:crypto';
 import { createRequire } from 'node:module';
 import { homedir, hostname } from 'node:os';
-import { join, dirname } from 'node:path';
+import { join, dirname, resolve } from 'node:path';
 import type { Command } from 'commander';
 import {
   buildResolvedFleetCommsBlock,
@@ -29,6 +29,7 @@ import {
 import { readRegularFileSecure } from '../fleet/secure-file.js';
 import { readPersonaContractBlock } from '../fleet/persona-contract.js';
 import { canonicalizeRoleClass } from './fleet-personas.js';
+import { resolveBrainHome } from '../fleet/brain-home.js';
 import { launchClaudex, type ClaudexHarnessAdapter } from './claudex.js';
 import { runLeaseEnforcementDoctorCheck } from './lease-doctor-check.js';
 
@@ -64,9 +65,46 @@ const HARNESS_HOME_ENV: Record<RuntimeName, string> = {
   opencode: 'XDG_CONFIG_HOME',
 };
 
-/** Dedicated mosaic-owned home for a runtime: ~/.config/mosaic/.<runtime> */
+/** Dedicated mosaic-owned home for a runtime: ~/.config/mosaic/.<runtime>.
-function harnessHome(runtime: RuntimeName): string {
+ * With an active brain seat (MOSAIC_AGENT_NAME + seat dir in the brain home)
-  return join(MOSAIC_HOME, `.${runtime}`);
+ * the home is per-agent instead: <brainHome>/fleet/agents/<seat>/.<runtime> —
+ * per-agent sessions, settings, and auth inside the seat dir (canon §2,
+ * MOSAIC-D-002). Seat runtime dirs are dot-named so the brain's ignore policy
+ * (per-seat .pi/.claude/.codex dirs) keeps credential material untracked. */
+const SEAT_AGENT_NAME_RE = /^[a-zA-Z0-9][a-zA-Z0-9._-]*$/;
+
+export function activeSeatDir(mosaicHome: string = MOSAIC_HOME): string | undefined {
+  const agent = process.env['MOSAIC_AGENT_NAME']?.trim();
+  if (
+    agent === undefined ||
+    agent === '' ||
+    !SEAT_AGENT_NAME_RE.test(agent) ||
+    agent.includes('..')
+  ) {
+    return undefined;
+  }
+  const brain = resolveBrainHome(mosaicHome);
+  if (resolve(brain) === resolve(mosaicHome)) return undefined; // no brain
+  const seat = join(brain, 'fleet', 'agents', agent);
+  return existsSync(seat) ? seat : undefined;
+}
+
+function harnessHome(runtime: RuntimeName, mosaicHome: string = MOSAIC_HOME): string {
+  const seat = activeSeatDir(mosaicHome);
+  if (seat !== undefined) return join(seat, `.${runtime}`);
+  return join(mosaicHome, `.${runtime}`);
+}
+
+/** Seat persona block: with an active brain seat, <seat>/SOUL.md layers
+ * persona on the root generic base (canon invariant; MOSAIC-D-002). The base
+ * SOUL stays load-on-demand — only the seat delta is injected by value.
+ * Empty string when no seat is active or the seat carries no SOUL.md. */
+export function seatPersonaOverlay(mosaicHome: string = MOSAIC_HOME): string {
+  const seatDir = activeSeatDir(mosaicHome);
+  if (seatDir === undefined) return '';
+  const seatSoul = readOptional(join(seatDir, 'SOUL.md'));
+  if (!seatSoul.trim()) return '';
+  return '## Seat Persona\n\n' + seatSoul.trim();
 }
 
 /**
@@ -182,6 +220,8 @@ function recordLaunch(runtime: RuntimeName, cliArgs: string[], yolo: boolean): v
       cli_version: CLI_VERSION,
       config_home: harnessHome(runtime),
       config_home_isolated: true,
+      config_home_kind: activeSeatDir() !== undefined ? 'seat' : 'runtime-shared',
+      agent_name: process.env['MOSAIC_AGENT_NAME']?.trim() || null,
       config_home_env: HARNESS_HOME_ENV[runtime] ?? null,
       argv: redactArgv(cliArgs),
       normative_fragments: normativeFragmentDigests(runtime),
@@ -569,6 +609,11 @@ For required push/merge/issue-close/release actions, execute without routine con
   if (soulLocal.trim()) {
     overlayBlocks.push('## Persona Overlay (SOUL.local.md)\n\n' + soulLocal.trim());
   }
+  // Seat persona (MOSAIC-D-002): per-seat SOUL.md layers on the generic base.
+  const seatPersona = seatPersonaOverlay(mosaicHome);
+  if (seatPersona !== '') {
+    overlayBlocks.push(seatPersona);
+  }
   const standardsLocal = readOptional(join(mosaicHome, 'STANDARDS.local.md'));
   if (standardsLocal.trim()) {
     overlayBlocks.push('## Standards Overlay (STANDARDS.local.md)\n\n' + standardsLocal.trim());