feat: replies to comments

A comment can answer another one (replyTo), one level deep: answering a
reply joins the same thread, and it must be on the same ticket. Whoever
wrote the comment gets a new REPLIED notification, and the activity says
"respondió a un comentario".

A comment deleted while it has replies stays, without its text
(deleted: true), so the thread still reads; it goes with its last reply
and does not count in commentCount.

The ticket page shows threads with their replies indented and a
"Responder" box; REST, GraphQL and the MCP (add_comment, get_issue) take
and show replyTo. Existing comments are unchanged. Docs, Word and PDF
updated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/api/tracker/application/src/lib/notifications/notifications.spec.ts

@@ -12,6 +12,7 @@ import {
 } from '@task-stream/api/tracker/domain';
 import { FakeUserDirectory } from '../../testing/in-memory';
 import { ChangeFeed } from '../ports/change-feed';
+import { FindCommentsRepository } from '../ports/find-comments.repository';
 import { FindIssuesRepository } from '../ports/find-issues.repository';
 import { FindProjectsRepository } from '../ports/find-projects.repository';
 import { NotificationMailer } from '../ports/notification-mailer';
@@ -100,6 +101,13 @@ function setup(mailEnabled = false) {
       }),
     } as unknown as FindProjectsRepository,
     users,
+    {
+      // A comment of the PM's, that others answer.
+      findById: async (id: string) =>
+        id === 'c-pm'
+          ? { id, author: { id: 'pm', name: 'PM' }, deleted: false }
+          : null,
+    } as unknown as FindCommentsRepository,
   );
   return { repo, published, mailed, handler };
 }
@@ -163,6 +171,7 @@ describe('notifications', () => {
         projectId: 'p-1',
         authorId: 'bob',
         body: 'Ya está arreglado',
+        replyTo: null,
         deleted: false,
         createdAt: '2026-10-08T10:00:00.000Z',
         editedAt: null,
@@ -185,6 +194,7 @@ describe('notifications', () => {
           projectId: 'p-1',
           authorId: 'bob',
           body: '@[PM](pm) mira esto, y @[Fuera](stranger) también',
+          replyTo: null,
           deleted: false,
           createdAt: '2026-10-08T10:00:00.000Z',
           editedAt: null,
@@ -200,6 +210,32 @@ describe('notifications', () => {
     expect(repo.all[0].summary).toContain('«@PM mira esto');
   });
 
+  it('tells whoever wrote a comment that someone answered it', async () => {
+    const { handler, repo } = setup();
+    await handler.handle(
+      at(
+        new CommentAddedEvent('bob', {
+          id: 'c-3',
+          issueId: 'i-1',
+          projectId: 'p-1',
+          authorId: 'bob',
+          body: 'Lo miro',
+          replyTo: 'c-pm',
+          deleted: false,
+          createdAt: '2026-10-08T10:00:00.000Z',
+          editedAt: null,
+        }),
+        1,
+      ),
+    );
+
+    expect(repo.all.map((n) => [n.userId, n.kind])).toEqual([
+      ['pm', NotificationKind.REPLIED],
+      ['qa', NotificationKind.COMMENTED],
+    ]);
+    expect(repo.all[0].summary).toContain('respondió a tu comentario');
+  });
+
   it('copies by e-mail only to whoever asked for it, when mail is configured', async () => {
     const { handler, repo, mailed } = setup(true);
     repo.emailOn.add('bob');