FN-6244: protect active AI merge worktrees

Ensure AI merge cleanup only reaps stale, inactive temp worktrees.

- Register live AI merge clean-room paths in the active session registry while merges run.
- Add a minimum temp-worktree reap age and conservative lookup-error handling to sweep and prune paths.
- Expand cleanup tests and release the repaired AI merge cleanup test from quarantine.

Files changed:
 docs/architecture.md                               |  2 +-
 .../engine/src/__tests__/merger-ai-cleanup.test.ts | 38 ++++++++++++++--
 .../__tests__/self-healing-tempdir-sweep.test.ts   | 52 +++++++++++++++++++---
 packages/engine/src/active-session-registry.ts     |  2 +-
 packages/engine/src/merger-ai.ts                   | 29 +++++++++++-
 packages/engine/src/self-healing.ts                | 28 +++++++++---
 packages/engine/vitest.config.ts                   |  1 -
 scripts/lib/test-quarantine.json                   |  5 ---
 8 files changed, 132 insertions(+), 25 deletions(-)

Fusion-Task-Id: FN-6244

Fusion-Task-Lineage: 5c8ced12-d38f-4e45-9ea0-119b3236a2cb

scripts/lib/test-quarantine.json

@@ -11,11 +11,6 @@
       "reason": "Flake: vi.mock('node:child_process') occasionally doesn't take under workspace-concurrent runs, letting real git binary leak and report staged files unrelated to test scope (trips FileScopeViolationError). Same logic covered by real-git fixture tests in reliability-interactions/workflow-and-file-scope. FN-6206.",
       "quarantinedAt": "2026-06-10"
     },
-    {
-      "file": "packages/engine/src/__tests__/merger-ai-cleanup.test.ts",
-      "reason": "Flake observed during FN-6206 verification: `pruneExistingAiMergeWorktrees skips active-session paths` failed in full `pnpm --filter @fusion/engine test` runs while the file passed standalone, indicating suite-order/concurrency sensitivity. Follow-up FN-6207.",
-      "quarantinedAt": "2026-06-10"
-    },
     {
       "file": "packages/engine/src/__tests__/self-healing-already-merged.real-git.test.ts",
       "reason": "Flake observed during FN-6226 verification: full `pnpm --filter @fusion/engine test` expected two run-audit events but saw four after unrelated real-git/self-healing cleanup activity. The failure is outside fast-mode workflow changes and indicates suite-order/temp-state sensitivity.",