FN-6016: quarantine flaky chat-manager generation-slot test

Quarantine the flaky dashboard chat-manager generation-slot test until the underlying race is fixed.

- exclude packages/dashboard/src/__tests__/chat-manager.test.ts from dashboard API Vitest projects
- record the quarantine entry with failure context, rescue requirements, and the 2026-06-08 quarantine date

Files changed:
 packages/dashboard/vitest.config.ts | 2 ++
 scripts/lib/test-quarantine.json    | 8 +++++++-
 2 files changed, 9 insertions(+), 1 deletion(-)

Fusion-Task-Id: FN-6016

Fusion-Task-Lineage: 2cd97f8e-db06-4e49-b437-66fee64fb3a9

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"
+    }
+  ]
 }