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/commands/add-comment.command-handler.ts

@@ -1,4 +1,5 @@
 import { CommandHandler, EventPublisher, ICommandHandler } from '@nestjs/cqrs';
+import { BusinessRuleError } from '@task-stream/api/shared/domain';
 import { CommentFactory } from '@task-stream/api/tracker/domain';
 import { AddCommentCommand } from './add-comment.command';
 import { CommandResult, ack } from './command-result';
@@ -24,10 +25,24 @@ export class AddCommentCommandHandler
       projectId: issue.projectId,
       authorId: command.actorId,
       body: command.body,
+      replyTo: command.replyTo
+        ? await this.threadOf(command.replyTo, issue.id)
+        : null,
     });
 
     this.eventPublisher.mergeObjectContext(comment);
     await comment.commitAndWait();
     return ack(comment);
   }
+
+  /**
+   * The top-level comment a reply hangs from: one level only, so answering
+   * a reply joins its thread.
+   */
+  private async threadOf(commentId: string, issueId: string): Promise<string> {
+    const parent = await this.access.comment(commentId);
+    if (parent.issueId !== issueId)
+      throw new BusinessRuleError('A reply must be on the same ticket');
+    return parent.replyTo ?? parent.id;
+  }
 }