feat(FN-3003): merge fusion/fn-3003

- Unify notification service ownership across the engine: `Notifier` now operates on `ProjectEngine` rather than its own `Core` reference, resolving stale-closure issues during engine lifecycle
- Add regression tests in `notifier.test.ts` covering merge dedupe wiring and ownership initialization
- Expand `project-engine.test.ts` to cover ownership state transitions and `ProjectEngine`-notifier integration
- Fix TypeScript regressions in `ModelOnboardingModal.tsx` and `SettingsModal.tsx` introduced by custom provider types
- Document the notification ownership model in `docs/architecture.md`

Commits merged:
- feat(FN-3003): complete Step 4 — document notification ownership model
- fix(FN-3003): resolve dashboard custom provider typecheck regressions
- test(FN-3003): complete Step 2 — add merge dedupe wiring regressions
- feat(FN-3003): complete Step 1 — unify notification service ownership

Files changed:
docs/architecture.md                               |  2 +
 .../app/components/ModelOnboardingModal.tsx        | 13 +++++-
 .../dashboard/app/components/SettingsModal.tsx     | 17 +++++--
 packages/engine/src/__tests__/notifier.test.ts     | 33 ++++++++++++++
 .../engine/src/__tests__/project-engine.test.ts    | 53 +++++++++++++++++++---
 packages/engine/src/notifier.ts                    |  3 +-
 packages/engine/src/project-engine.ts              | 12 +++--
 7 files changed, 116 insertions(+), 17 deletions(-)

Fusion-Task-Id: FN-3003

docs/architecture.md

@@ -339,6 +339,8 @@ See [Memory Plugin Contract](./memory-plugin-contract.md) for the full plan.
 - `AgentLogger` (`agent-logger.ts`) — structured per-agent run logging
 - `RunAudit` (`run-audit.ts`) — mutation audit tracking (DB/git/filesystem)
 - `Notifier` (`notifier.ts`) — legacy ntfy compatibility shim (`NtfyNotifier`) plus shared ntfy helpers
+  - Runtime ownership: `NtfyNotifier` no longer owns an independent task-lifecycle listener graph; `ProjectEngine` injects the canonical `NotificationService` instance so task lifecycle notifications (`task:moved`, `task:updated`, `task:merged`) are emitted through a single path.
+  - Compatibility scope: `NtfyNotifier` remains responsible for gridlock-only compatibility notifications (`notifyGridlock`) and legacy helper APIs.
 - `NotificationService` (`notification/notification-service.ts`) — provider lifecycle + event dispatch orchestration
 - `NotificationProvider` interface (`@fusion/core` `notification/provider.ts`) — pluggable provider contract
 - Built-in providers: `NtfyNotificationProvider` (`notification/ntfy-provider.ts`), `WebhookNotificationProvider` (`notification/webhook-provider.ts`)