FN-6195: isolate and quarantine flaky db-corruption test

Stop the self-healing DB corruption test from leaking into unrelated tmpdir state while quarantining the known flake.

- mock node:os tmpdir usage so the test runs inside a per-test sandbox and cleans it up after each case
- add the flaky self-healing DB corruption test to the engine default exclude list
- record the quarantine entry and failure context in scripts/lib/test-quarantine.json

Files changed:
 .../__tests__/self-healing-db-corruption.test.ts   | 22 ++++++++++++++++++++++
 packages/engine/vitest.config.ts                   |  1 +
 scripts/lib/test-quarantine.json                   |  8 +++++++-
 3 files changed, 30 insertions(+), 1 deletion(-)

Fusion-Task-Id: FN-6195

Fusion-Task-Lineage: 60f7f6ca-d14d-42b4-b47e-0644a25e64ae

scripts/lib/test-quarantine.json

@@ -1,4 +1,10 @@
 {
   "$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": []
+  "entries": [
+    {
+      "file": "packages/engine/src/__tests__/self-healing-db-corruption.test.ts",
+      "reason": "FN-6195: observed flake when runMaintenance() scanned the real OS tmpdir and picked up stale fusion-ai-merge-* worktrees from unrelated tests, adding unexpected audit calls. Failing run: Fusion task FN-6195 agent verification log, 2026-06-10.",
+      "quarantinedAt": "2026-06-10"
+    }
+  ]
 }