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/domain/src/lib/issue/issue.spec.ts

@@ -8,6 +8,7 @@ import { IssueStatusChangedEvent } from '../events/issue-status-changed.event';
 import { IssueFactory } from '../factories/issue.factory';
 import { replay } from '../testing';
 import { Issue } from './issue';
+import { mentionedUserIds, withoutMentionMarkup } from './mentions';
 import { IssueLinkType, IssueStatus, IssueType } from './issue.enums';
 
 const factory = new IssueFactory();
@@ -206,4 +207,31 @@ describe('Issue', () => {
       expect(rebuilt.attachments.map((a) => a.id)).toEqual(['a-2']);
     });
   });
+
+  describe('followers', () => {
+    it('follows and unfollows once, and survives a replay', () => {
+      const issue = newIssue();
+      issue.watch('u-dev', 'u-dev');
+      issue.watch('u-dev', 'u-dev');
+      issue.watch('u-pm', 'u-pm');
+      issue.unwatch('u-dev', 'u-dev');
+      issue.unwatch('u-dev', 'u-dev');
+
+      expect(issue.watchers).toEqual(['u-pm']);
+      expect(replay(issue, Issue).rebuilt.watchers).toEqual(['u-pm']);
+      expect(issue.getUncommittedEvents()).toHaveLength(4);
+    });
+  });
+
+  describe('mentions', () => {
+    it('reads who a text mentions, once each', () => {
+      const text =
+        'Hola @[Ana García](u-ana) y @[Bob](u-bob), @[Ana García](u-ana) otra vez. @sinmarca';
+
+      expect(mentionedUserIds(text)).toEqual(['u-ana', 'u-bob']);
+      expect(withoutMentionMarkup(text)).toBe(
+        'Hola @Ana García y @Bob, @Ana García otra vez. @sinmarca',
+      );
+    });
+  });
 });