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/domain/src/lib/read-models/comment.read-model.ts

@@ -6,6 +6,13 @@ export class CommentReadModel {
   projectId!: string;
   author!: UserRefReadModel;
   body!: string;
+  /** Top-level comment it answers; null for a top-level one. */
+  replyTo!: string | null;
+  /**
+   * Deleted while it had replies: kept, without its text, so the
+   * conversation under it still makes sense. Gone with its last reply.
+   */
+  deleted!: boolean;
   createdAt!: string;
   editedAt!: string | null;
   version!: number;