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/presenters/src/lib/graphql/comment.resolver.ts

@@ -8,13 +8,18 @@ import { CommandResultGql, CommentGql } from './types';
 export class CommentResolver {
   constructor(private readonly comments: CommentsService) {}
 
-  @Mutation(() => CommandResultGql)
+  @Mutation(() => CommandResultGql, {
+    description:
+      'Comment on a ticket, or answer a comment (replyTo; one level of replies)',
+  })
   addComment(
     @CurrentUser() me: User,
     @Args('issueId', { type: () => ID }) issueId: string,
     @Args('body') body: string,
+    @Args('replyTo', { type: () => ID, nullable: true })
+    replyTo: string | null,
   ) {
-    return this.comments.add(me.id, issueId, body);
+    return this.comments.add(me.id, issueId, body, replyTo ?? null);
   }
 
   @Mutation(() => CommandResultGql)

libs/api/tracker/presenters/src/lib/graphql/types.ts

@@ -445,7 +445,17 @@ export class CommentGql {
   @Field(() => ID) id!: string;
   @Field(() => ID) issueId!: string;
   @Field(() => UserRefGql) author!: UserRefGql;
-  @Field() body!: string;
+  @Field({ description: 'Empty on a deleted comment kept for its replies' })
+  body!: string;
+  @Field(() => ID, {
+    nullable: true,
+    description: 'The top-level comment it answers; null for a top-level one',
+  })
+  replyTo!: string | null;
+  @Field({
+    description: 'Deleted, but kept (without its text) while it has replies',
+  })
+  deleted!: boolean;
   @Field() createdAt!: string;
   @Field(() => String, { nullable: true }) editedAt!: string | null;
   @Field(() => Int) version!: number;