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/web/tracker/domain/src/index.ts

@@ -3,3 +3,4 @@ export * from './lib/models/models';
 export * from './lib/models/inputs';
 export * from './lib/models/workflow';
 export * from './lib/ports/gateways';
+export * from './lib/models/mentions';