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/presenters/src/lib/graphql/issue.resolver.ts

@@ -183,6 +183,25 @@ export class IssueResolver {
     );
   }
 
+  @Mutation(() => CommandResultGql, {
+    description:
+      'Follow a ticket: be told when it moves, is reassigned or gets a comment',
+  })
+  watchIssue(
+    @CurrentUser() me: User,
+    @Args('issueId', { type: () => ID }) issueId: string,
+  ) {
+    return this.issuesService.watch(me.id, issueId, true);
+  }
+
+  @Mutation(() => CommandResultGql, { description: 'Stop following a ticket' })
+  unwatchIssue(
+    @CurrentUser() me: User,
+    @Args('issueId', { type: () => ID }) issueId: string,
+  ) {
+    return this.issuesService.watch(me.id, issueId, false);
+  }
+
   @Mutation(() => CommandResultGql, {
     description:
       'Remove a file from a ticket (whoever uploaded it, or an owner/admin). Upload with the REST API.',