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/comment/comment.ts

@@ -15,6 +15,12 @@ export interface CommentProps {
   projectId: string;
   authorId: string;
   body: string;
+  /**
+   * The comment this one answers; always a top-level one (one level of
+   * replies). Null for a top-level comment, and on comments made before
+   * there were replies.
+   */
+  replyTo: string | null;
   deleted: boolean;
   createdAt: string;
   editedAt: string | null;
@@ -22,12 +28,13 @@ export interface CommentProps {
 
 const MAX_BODY = 20_000;
 
-/** Decisions and explanations inside a ticket. */
+/** Decisions and explanations inside a ticket, and the replies to them. */
 export class Comment extends VersionedAggregateRoot implements CommentProps {
   public issueId!: string;
   public projectId!: string;
   public authorId!: string;
   public body!: string;
+  public replyTo: string | null = null;
   public deleted = false;
   public createdAt!: string;
   public editedAt: string | null = null;