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/index.ts

@@ -24,3 +24,5 @@ export * from './issue-deleted.event';
 export * from './release-deleted.event';
 export * from './issue-attachment-added.event';
 export * from './issue-attachment-removed.event';
+export * from './issue-watched.event';
+export * from './issue-unwatched.event';