FN-6012: quarantine flaky dashboard chat manager test

Quarantine the flaky dashboard chat manager test and keep it out of active API lanes.

- record packages/dashboard/src/__tests__/chat-manager.test.ts in the quarantine ledger with the current failure context and quarantine date
- exclude the quarantined chat-manager test from dashboard API backfill coverage so existing missions surface an explicit no-backfill decision
- exclude the quarantined chat-manager test from the broad dashboard API project so its status stays visible without destabilizing the suite

Files changed:
 packages/dashboard/vitest.config.ts | 3 ++-
 scripts/lib/test-quarantine.json    | 8 +++++++-
 2 files changed, 9 insertions(+), 2 deletions(-)

Fusion-Task-Id: FN-6012

Fusion-Task-Lineage: 34014c45-486e-443d-ae47-a1d301a396e1

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/dashboard/src/__tests__/chat-manager.test.ts",
+      "reason": "Flaky: 'old generation finally does not delete a newer generation's slot' times out under full-suite load. Prior fix attempts (FN-4012 on 2026-05-11, FN-5982 on 2026-06-06 which exhausted its stuck-kill budget 10/6 and was manually paused twice) failed to address the underlying generation-slot race. Per standing rule: quarantine on sight, not appease. Rescue requires (a) evidence the test catches real regressions and (b) a root-cause fix for the generation-slot race — not another synchronization tweak.",
+      "quarantinedAt": "2026-06-08"
+    }
+  ]
 }