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/web/tracker/domain/src/lib/models/models.ts

@@ -269,7 +269,12 @@ export interface Comment {
   id: string;
   issueId: string;
   author: UserRef;
+  /** Empty on a deleted comment kept for its replies. */
   body: string;
+  /** The top-level comment it answers; null for a top-level one. */
+  replyTo: string | null;
+  /** Deleted, but kept (without its text) while it has replies. */
+  deleted: boolean;
   createdAt: string;
   editedAt: string | null;
 }