feat(api): wire notification dispatcher into app + /notifications routes

PermissionManager: add onPromptAdded(listener) callback. Fires exactly
once per unique pending prompt id, even when broadcastPending is called
repeatedly for unrelated mutations (e.g. another prompt resolving while
this one is still pending).

app.ts: instantiate NotificationDispatcher, attach to both AgentManager
and PermissionManager. Tab-title lookup via core's getTab so the
notifications carry human-readable context instead of raw UUIDs.

routes/notifications.ts:
- GET  /notifications      — current config (auth token redacted) plus
                             the event-type catalog and defaults
- PUT  /notifications      — partial update; auth token semantics are
                             undefined=keep, ''=clear, otherwise replace
- POST /notifications/test — sends a test notification with the current
                             config (rejects if disabled or topic invalid)

Tests:
- new permission-manager.test.ts covers the onPromptAdded contract
  (one-fire-per-prompt, dedup across rebroadcasts, unsubscribe, listener
  throws don't break siblings)
- existing routes.test.ts gets stubs for the new core notification
  exports so the @dispatch/core mock stays complete

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