FN-6486: rescue quarantined flaky tests

Rescue the same-day core and CLI flaky quarantines by fixing their fixture seams instead of appeasing timeouts.

- Make the core concurrent-write lock helper release synchronously inside its child process so package load cannot delay the transient lock release.
- Close real CLI TaskStore fixtures before removing temp roots and switch mock cleanup to non-hoisted unmocking.
- Remove both test files from package quarantine excludes and clear the quarantine ledger while documenting the rescue pattern.

Files changed:
 docs/testing.md                                         |  2 ++
 packages/cli/src/__tests__/extension-task-tools.test.ts | 17 +++++++++++++----
 packages/cli/vitest.config.ts                           |  4 +++-
 .../core/src/__tests__/store-concurrent-writes.test.ts  |  8 +++++++-
 packages/core/vitest.config.ts                          |  4 +++-
 scripts/lib/test-quarantine.json                        | 13 +------------
 6 files changed, 29 insertions(+), 19 deletions(-)

Fusion-Task-Id: FN-6486

Fusion-Task-Lineage: c90358c4-0549-4c68-8d17-2e2a336433b5

docs/testing.md

@@ -160,6 +160,8 @@ Legitimate legacy exceptions must be recorded in `scripts/lib/test-timeout-appea
 
 **Non-CLI quarantine sweep pattern (FN-6433):** for engine/core/dashboard batches, first remove quarantine excludes only in temporary local configs and run the exact quarantined files together so suite-load coupling is visible before editing the ledger. Rescue is valid when the grouped package lane proves the invariant now holds (for example, FN-6433 fixed engine cross-file interference by replacing broad `activeSessionRegistry.clear()` cleanup with path-scoped unregistering) or when a prior shared-fixture fix is demonstrated under package load. Delete duplicate/low-value files under the ratchet when another deterministic suite owns the same invariant. Finish by making `scripts/lib/test-quarantine.json` and every package Vitest exclude array converge in one commit, then prove the empty/non-empty state with package lanes, `pnpm test:gate`, `pnpm test`, `pnpm build`, and the bounded temp-leak output from `pnpm test`.
 
+**2026-06-15 rescue batch (FN-6486):** two same-day quarantines were rescued before their 2026-06-29 deletion deadline. `store-concurrent-writes.test.ts` kept its WAL/`transactionImmediate` regression value by making the external lock helper's timed release use synchronous `Atomics.wait` inside the child process, removing event-loop timer scheduling as the load-only flake source without widening retry windows. `extension-task-tools.test.ts` kept its worktree-root task-tool coverage by closing each real `TaskStore` fixture before temp-root removal and using non-hoisted mock cleanup. The reusable pattern is to remove scheduler/resource leaks in the helper or fixture seam, then prove the rescue with repeated exact-file runs plus package lanes, not with timeout bumps, retries, assertion loosening, or worker changes.
+
 **Gate eviction:** a flake inside the merge gate cannot block all merges while red — it is evicted by removing its line from the `engine-core` allow-list (no quarantine entry needed unless it should also leave the non-blocking tier).
 
 **Gate admission:** the mirror operation — add the test's path to the `engine-core` `include` array in `packages/engine/vitest.config.ts`, citing the evidence of value (a real regression it caught) in the PR. Keep the project under its ~60s wall-clock budget.