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/application/src/lib/facades/issue.facade.ts

@@ -30,6 +30,8 @@ export class IssueFacade {
   readonly error = this.store.error;
   /** Signed-in user's id (authors can edit their comments). */
   readonly me = this.store.me;
+  /** Whether the signed-in person follows the open ticket. */
+  readonly watching = this.store.watching;
 
   /** Loads the ticket (by id or key, e.g. BE-3) and follows its changes. */
   open(idOrKey: string): void {
@@ -85,6 +87,13 @@ export class IssueFacade {
     );
   }
 
+  /** Follow the ticket (or stop): be told when it moves or gets a comment. */
+  setWatching(watching: boolean): void {
+    this.withIssue((issueId) =>
+      this.dispatch.watchRequested({ issueId, watching }),
+    );
+  }
+
   /** Attaches files to the ticket (or under one of its comments). */
   attach(files: File[], commentId: string | null = null): void {
     if (!files.length) return;