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/sagas/commenters-follow.saga.spec.ts

@@ -0,0 +1,25 @@
+import { CommentAddedEvent } from '@task-stream/api/tracker/domain';
+import { lastValueFrom, of, toArray } from 'rxjs';
+import { WatchIssueCommand } from '../commands/watch-issue.command';
+import { CommentersFollowSaga } from './commenters-follow.saga';
+
+describe('CommentersFollowSaga', () => {
+  it('makes whoever comments follow the ticket', async () => {
+    const event = new CommentAddedEvent('ana', {
+      id: 'c-1',
+      issueId: 'i-1',
+      projectId: 'p-1',
+      authorId: 'ana',
+      body: 'Lo miro',
+      deleted: false,
+      createdAt: '2026-10-08T10:00:00.000Z',
+      editedAt: null,
+    });
+
+    const commands = await lastValueFrom(
+      new CommentersFollowSaga().commented(of(event)).pipe(toArray()),
+    );
+
+    expect(commands).toEqual([new WatchIssueCommand('ana', 'i-1', 'ana')]);
+  });
+});