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.ts

@@ -14,6 +14,8 @@ import { IssueReleaseChangedEvent } from '../events/issue-release-changed.event'
 import { IssueStatusChangedEvent } from '../events/issue-status-changed.event';
 import { IssueUnlinkedEvent } from '../events/issue-unlinked.event';
 import { IssueUpdatedEvent } from '../events/issue-updated.event';
+import { IssueUnwatchedEvent } from '../events/issue-unwatched.event';
+import { IssueWatchedEvent } from '../events/issue-watched.event';
 import { ProjectWorkflow } from '../project/project-settings';
 import {
   IssueAttachment,
@@ -91,6 +93,8 @@ export class Issue extends VersionedAggregateRoot implements IssueProps {
   public dueDate: string | null = null;
   public links: IssueLink[] = [];
   public attachments: IssueAttachment[] = [];
+  /** People who asked to be told about this ticket. */
+  public watchers: string[] = [];
   public createdAt!: string;
   public updatedAt!: string;
   public deleted = false;
@@ -262,6 +266,20 @@ export class Issue extends VersionedAggregateRoot implements IssueProps {
     );
   }
 
+  /** `userId` starts following the ticket. Idempotent. */
+  watch(actorId: string, userId: string): void {
+    if (this.watchers.includes(userId)) return;
+    this.apply(new IssueWatchedEvent(actorId, this.id, this.projectId, userId));
+  }
+
+  /** `userId` stops following the ticket. Idempotent. */
+  unwatch(actorId: string, userId: string): void {
+    if (!this.watchers.includes(userId)) return;
+    this.apply(
+      new IssueUnwatchedEvent(actorId, this.id, this.projectId, userId),
+    );
+  }
+
   /** Deletes the ticket (its project was deleted). Idempotent. */
   delete(actorId: string): void {
     if (this.deleted) return;
@@ -334,6 +352,14 @@ export class Issue extends VersionedAggregateRoot implements IssueProps {
     this.updatedAt = event.occurredAt;
   }
 
+  onIssueWatchedEvent(event: IssueWatchedEvent): void {
+    this.watchers = [...this.watchers, event.userId];
+  }
+
+  onIssueUnwatchedEvent(event: IssueUnwatchedEvent): void {
+    this.watchers = this.watchers.filter((id) => id !== event.userId);
+  }
+
   onIssueDeletedEvent(): void {
     this.deleted = true;
   }