FN-5795: send ntfy notification when agents create tasks

Add agent task-created ntfy notifications with configurable settings and coverage.

- add notification plumbing to emit a dedicated event when an agent creates a task
- extend ntfy provider/settings typing and dashboard settings coverage for the new event toggle
- add regression tests for notification service, notifier integration, and ntfy provider behavior
- add a changeset and documentation updates describing the new notification capability

Files changed:
 .changeset/fn-5795-task-created-notification.md    |  9 +++
 docs/architecture.md                               |  4 +-
 docs/settings-reference.md                         |  4 +-
 docs/storage.md                                    |  2 +-
 packages/core/src/types.ts                         |  4 +-
 .../dashboard/app/components/SettingsModal.tsx     |  1 +
 .../components/__tests__/SettingsModal.test.tsx    | 27 +++++++-
 .../src/__tests__/notification-service.test.ts     | 71 ++++++++++++++++++++++
 packages/engine/src/__tests__/notifier.test.ts     |  5 ++
 .../engine/src/__tests__/ntfy-provider.test.ts     | 32 ++++++++++
 .../src/notification/notification-service.ts       | 53 +++++++++++++++-
 packages/engine/src/notification/ntfy-provider.ts  | 21 +++++--
 12 files changed, 220 insertions(+), 13 deletions(-)

Fusion-Task-Id: FN-5795

Fusion-Task-Lineage: 4cfa3b46-8e9c-451e-ab2d-b7d5c9bf5968

packages/core/src/types.ts

@@ -511,7 +511,8 @@ export type NtfyNotificationEvent =
   | "message:agent-to-user"
   | "message:agent-to-agent"
   | "message:room"
-  | "oauth-token-expired";
+  | "oauth-token-expired"
+  | "task-created";
 
 /** Known notification event types. Providers may support additional custom events. */
 export const NOTIFICATION_EVENTS = [
@@ -531,6 +532,7 @@ export const NOTIFICATION_EVENTS = [
   "message:agent-to-agent",
   "message:room",
   "oauth-token-expired",
+  "task-created",
 ] as const;
 
 /** Notification event type. Known events plus provider-specific custom events. */