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/issues.service.ts

@@ -12,6 +12,7 @@ import {
   IncomingFile,
 } from './commands/attach-file.command';
 import { DetachFileCommand } from './commands/detach-file.command';
+import { WatchIssueCommand } from './commands/watch-issue.command';
 import { ChangeIssueStatusCommand } from './commands/change-issue-status.command';
 import { CommandResult } from './commands/command-result';
 import { CreateIssueCommand } from './commands/create-issue.command';
@@ -157,6 +158,17 @@ export class IssuesService {
     );
   }
 
+  /** The actor follows (or stops following) the ticket. */
+  watch(
+    actorId: string,
+    issueId: string,
+    watching: boolean,
+  ): Promise<CommandResult> {
+    return this.commandBus.execute(
+      new WatchIssueCommand(actorId, issueId, actorId, watching),
+    );
+  }
+
   /** A ticket's file, with its bytes, for whoever can read the ticket. */
   download(
     viewerId: string | null,