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.ts

@@ -6,6 +6,8 @@ export class AddCommentCommand extends Command<CommandResult> {
     public readonly actorId: string,
     public readonly issueId: string,
     public readonly body: string,
+    /** Comment it answers; a reply to a reply joins the same thread. */
+    public readonly replyTo: string | null = null,
   ) {
     super();
   }