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/commands.spec.ts

@@ -40,6 +40,7 @@ import { AddProjectMemberCommand } from './add-project-member.command';
 import { AssignIssueCommand } from './assign-issue.command';
 import { AttachFileCommand } from './attach-file.command';
 import { DetachFileCommand } from './detach-file.command';
+import { WatchIssueCommand } from './watch-issue.command';
 import { ChangeIssueStatusCommand } from './change-issue-status.command';
 import { COMMAND_HANDLERS } from './command-handlers';
 import { CommandResult } from './command-result';
@@ -258,6 +259,28 @@ describe('tracker commands (event sourced)', () => {
     });
   });
 
+  it('lets any member, viewers included, follow a ticket for themselves', async () => {
+    const { run, rehydrator } = await setup();
+    const projectId = await projectWithTeam(run);
+    const issue = await run(
+      new CreateIssueCommand('qa', projectId, IssueType.BUG, 'Crash', {}),
+    );
+
+    await run(new WatchIssueCommand('pm', issue.id, 'pm'));
+    await run(new WatchIssueCommand('dev', issue.id, 'dev'));
+    await run(new WatchIssueCommand('dev', issue.id, 'dev', false));
+
+    expect((await rehydrator.rehydrate(issue.id, Issue)).watchers).toEqual([
+      'pm',
+    ]);
+    await expect(
+      run(new WatchIssueCommand('dev', issue.id, 'pm', false)),
+    ).rejects.toThrow(ForbiddenError);
+    await expect(
+      run(new WatchIssueCommand('gone', issue.id, 'gone')),
+    ).rejects.toThrow(ForbiddenError);
+  });
+
   it('does not let viewers modify tickets', async () => {
     const { run } = await setup();
     const projectId = await projectWithTeam(run);