FN-6742: rescue session cross-tab tests

Rescue the dashboard session cross-tab test before its deletion-ratchet deadline.

- Dispose the API router and session cleanup scheduler before closing test stores.
- Hide unused TaskStore EventEmitter hooks in the route-only harness to avoid background route subscribers reopening temp fixtures.
- Remove the session-cross-tab quarantine ledger entry and dashboard Vitest exclude in lockstep.
- Document the rescue evidence and teardown pattern for future dashboard session tests.

Files changed:
 docs/testing.md                                    |  6 ++++-
 .../src/__tests__/session-cross-tab.test.ts        | 30 +++++++++++++++++++---
 packages/dashboard/vitest.config.ts                |  5 +++-
 scripts/lib/test-quarantine.json                   |  5 ----
 4 files changed, 36 insertions(+), 10 deletions(-)

Fusion-Task-Id: FN-6742

Fusion-Task-Lineage: e26d70eb-f139-45f7-8ddd-bde2d172c291

scripts/lib/test-quarantine.json

@@ -1,11 +1,6 @@
 {
   "$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/dashboard/src/__tests__/session-cross-tab.test.ts",
-      "reason": "FN-6690 local workspace `pnpm test` observed ENOTEMPTY while removing the test's temp .fusion directory in dashboard-api-quality-backfill shard; isolated rerun passed, indicating cleanup flake rather than a lazy-view CSS regression.",
-      "quarantinedAt": "2026-06-19"
-    },
     {
       "file": "packages/dashboard/app/components/__tests__/QuickEntryBox.test.tsx",
       "reason": "FN-6697 local workspace `pnpm test` observed the post-submission focus restoration test fail in the broad dashboard app backfill shard, while a targeted rerun of QuickEntryBox with MailboxModal passed the QuickEntryBox assertions; quarantine the focus-timing flake instead of appeasing it while the terminal shortcut fix remains scoped.",