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/infrastructure/src/lib/mongo/find.repositories.ts

@@ -45,6 +45,13 @@ import {
 
 /* Query-side adapters (MongoDB). */
 
+/** Comments projected before there were replies have neither field. */
+const commentFromDoc = (doc: CommentDoc): CommentReadModel => ({
+  ...fromDoc(doc),
+  replyTo: doc.replyTo ?? null,
+  deleted: doc.deleted ?? false,
+});
+
 const escapeRegex = (text: string) =>
   text.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');
 
@@ -446,7 +453,7 @@ export class MongoFindCommentsRepository extends FindCommentsRepository {
 
   async findById(id: string): Promise<CommentReadModel | null> {
     const doc = await this.comments.findById(id).lean<CommentDoc>();
-    return doc ? fromDoc(doc) : null;
+    return doc ? commentFromDoc(doc) : null;
   }
 
   async findByIssue(
@@ -463,11 +470,15 @@ export class MongoFindCommentsRepository extends FindCommentsRepository {
         .lean<CommentDoc[]>(),
       this.comments.countDocuments({ issueId }),
     ]);
-    return { items: docs.map(fromDoc), total };
+    return { items: docs.map(commentFromDoc), total };
   }
 
   countByIssue(issueId: string): Promise<number> {
-    return this.comments.countDocuments({ issueId });
+    return this.comments.countDocuments({ issueId, deleted: { $ne: true } });
+  }
+
+  countReplies(commentId: string): Promise<number> {
+    return this.comments.countDocuments({ replyTo: commentId });
   }
 }