fix(agent-execution): quarantine malformed bus history and fail fan-out baseline loudly (AGT-3487)

[unohee]

Closes AGT-3487.

What

• AgentBus history quarantine (src/agents/agentBus.ts): getAllMessages() now shape-checks each JSON file (id/timestamp/type/sender/executionId as strings). Malformed or corrupt files are moved into a sibling quarantine/ directory (recoverable, inspectable) instead of throwing — one corrupt payload can no longer wedge the whole retrieval pass, and valid history keeps flowing.
• Auditor/Documenter non-string guard (src/agents/auditor.ts, src/agents/documenter.ts): parse functions now reject non-string outputs explicitly (transport defect) instead of feeding them into NDJSON splitting / JSON assumptions. parseAuditorOutput / parseDocumenterOutput are exported so the contract is testable.
• Fan-out baseline capture is fail-loud (src/agents/workerFanout.ts): when captureBaselinePatch throws (git broken, index race, FS error), fan-out now returns an explicit fallbackReason: 'baseline capture failed: …' instead of silently falling back to an empty baseline — which would have made the winner's diff base+delta and double-applied the dirty state on promotion. A genuinely clean worktree still yields an empty patch normally.

DoD mapping

• Malformed history files are quarantined without blocking valid history
• Auditor and documenter parse only string result payloads
• Fan-out captures a valid baseline or uses an explicit safe fallback/failure path
• Shared-path configuration cannot imply unsupported isolation — the existing sharedPathModeFor maps undefined → 'copy' (safe default), true → 'link', false → 'off'; link mode still goes through linkSharedPaths which never silently degrades isolation (verified by existing tests, no change needed)

Verification

• npm run typecheck — 0 errors
• vitest: agentBus + auditor + documenter + workerFanout suites + new artifactValidation.test.ts — 163 passed
• oxlint on touched files — 0 warnings