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/index.ts

@@ -3,6 +3,7 @@ export * from './lib/project/project-role';
 export * from './lib/issue/issue';
 export * from './lib/issue/issue.enums';
 export * from './lib/issue/issue-attachment';
+export * from './lib/issue/mentions';
 export * from './lib/project/project-settings';
 export * from './lib/comment/comment';
 export * from './lib/release/release';