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

scripts/lib/test-quarantine.json

@@ -1,15 +1,4 @@
 {
   "$comment": "Flaky-test quarantine ledger (deletion ratchet — see AGENTS.md 'Flaky tests: quarantine on sight' and docs/testing.md 'Quarantine ledger and the deletion ratchet'). A test observed failing without a corresponding real bug is quarantined ON SIGHT: add an entry here AND a matching one-line `exclude` entry in that package's vitest config, in the same commit. Every entry needs a non-empty `reason` (link the failing run) and a `quarantinedAt` date — the entry expires 14 days later, at which point the test file is DELETED unless someone rescues it with evidence it catches real regressions plus a root-cause fix (never appeasement). There is deliberately no loader module and no automation around this file: it is a dated record, the vitest config exclude is the mechanism, and the sweep is policy executed by whoever touches the suite.",
-  "entries": [
-    {
-      "file": "packages/core/src/__tests__/store-concurrent-writes.test.ts",
-      "reason": "FN-6481 pnpm test 2026-06-15 failed unrelated core SQLite lock recovery concurrency test with `SQLite BEGIN IMMEDIATE failed after 7 attempts: database is locked`; quarantine on sight per flaky-test policy.",
-      "quarantinedAt": "2026-06-15"
-    },
-    {
-      "file": "packages/cli/src/__tests__/extension-task-tools.test.ts",
-      "reason": "FN-6483: @runfusion/fusion package lane under load timed out after 5000ms in uses canonical project root for fn_task_show and fn_task_list from worktree cwd during the 2026-06-15 verification rerun, matching the FN-6482 reported load-only CLI timeout signature. The same file passed standalone immediately afterward (4 tests passed in 5.90s), so quarantine per the deletion ratchet rather than widening timeouts or changing worker knobs.",
-      "quarantinedAt": "2026-06-15"
-    }
-  ]
+  "entries": []
 }