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/comments.service.ts

@@ -15,9 +15,14 @@ export class CommentsService {
     private readonly queryBus: QueryBus,
   ) {}
 
-  add(actorId: string, issueId: string, body: string): Promise<CommandResult> {
+  add(
+    actorId: string,
+    issueId: string,
+    body: string,
+    replyTo: string | null = null,
+  ): Promise<CommandResult> {
     return this.commandBus.execute(
-      new AddCommentCommand(actorId, issueId, body),
+      new AddCommentCommand(actorId, issueId, body, replyTo),
     );
   }