feat: mentions and following tickets

Mention project members in comments with @, follow and unfollow tickets,
and get told when something changes on the tickets you follow. Commenting
on a ticket starts following it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/api/tracker/domain/src/lib/events/issue-watched.event.ts

@@ -0,0 +1,15 @@
+import { AutowiredEvent } from '@task-stream/api/shared/domain';
+import { TrackerEvent } from './tracker.event';
+
+/** Someone started following a ticket. */
+@AutowiredEvent
+export class IssueWatchedEvent extends TrackerEvent {
+  constructor(
+    actorId: string,
+    issueId: string,
+    projectId: string,
+    readonly userId: string,
+  ) {
+    super(actorId, issueId, projectId);
+  }
+}