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.event-handler.ts

@@ -11,6 +11,7 @@ import {
   TrackerEvent,
   withoutMentionMarkup,
 } from '@task-stream/api/tracker/domain';
+import { FindCommentsRepository } from '../ports/find-comments.repository';
 import { FindIssuesRepository } from '../ports/find-issues.repository';
 import { FindProjectsRepository } from '../ports/find-projects.repository';
 import { UserDirectory } from '../ports/user-directory';
@@ -43,6 +44,7 @@ export class NotificationsEventHandler implements IEventHandler<TrackerEvent> {
     private readonly findIssues: FindIssuesRepository,
     private readonly findProjects: FindProjectsRepository,
     private readonly users: UserDirectory,
+    private readonly findComments: FindCommentsRepository,
   ) {}
 
   async handle(event: TrackerEvent): Promise<void> {
@@ -154,6 +156,16 @@ export class NotificationsEventHandler implements IEventHandler<TrackerEvent> {
           summary: `${actor} te mencionó en ${issue.key}: ${said}`,
         });
       }
+      const answered = event.comment.replyTo
+        ? await this.findComments.findById(event.comment.replyTo)
+        : null;
+      if (answered && !answered.deleted) {
+        drafts.push({
+          userId: answered.author.id,
+          kind: NotificationKind.REPLIED,
+          summary: `${actor} respondió a tu comentario en ${issue.key}: ${said}`,
+        });
+      }
       for (const userId of [assigneeId, reporterId]) {
         if (!userId) continue;
         drafts.push({