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/application/src/lib/facades/issue.facade.ts

@@ -92,10 +92,14 @@ export class IssueFacade {
     );
   }
 
-  /** `files` are attached under the new comment. */
-  addComment(body: string, files: File[] = []): void {
+  /** `files` are attached under the new comment; `replyTo` answers one. */
+  addComment(
+    body: string,
+    files: File[] = [],
+    replyTo: string | null = null,
+  ): void {
     this.withIssue((issueId) =>
-      this.dispatch.commentAddRequested({ issueId, body, files }),
+      this.dispatch.commentAddRequested({ issueId, body, files, replyTo }),
     );
   }