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/application/src/lib/commands/command-handlers.ts

@@ -27,6 +27,7 @@ import { DetachIssueFromReleaseCommandHandler } from './detach-issue-from-releas
 import { UnassignIssueCommandHandler } from './unassign-issue.command-handler';
 import { AttachFileCommandHandler } from './attach-file.command-handler';
 import { DetachFileCommandHandler } from './detach-file.command-handler';
+import { WatchIssueCommandHandler } from './watch-issue.command-handler';
 
 export const COMMAND_HANDLERS = [
   CreateProjectCommandHandler,
@@ -59,4 +60,5 @@ export const COMMAND_HANDLERS = [
   UnassignIssueCommandHandler,
   AttachFileCommandHandler,
   DetachFileCommandHandler,
+  WatchIssueCommandHandler,
 ];