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/http/dto/responses.dto.ts

@@ -466,7 +466,21 @@ export class CommentDto {
   @ApiProperty({ format: 'uuid' }) id!: string;
   @ApiProperty({ format: 'uuid' }) issueId!: string;
   @ApiProperty({ type: UserRefDto }) author!: UserRefDto;
-  @ApiProperty() body!: string;
+  @ApiProperty({
+    description: 'Empty on a deleted comment kept for its replies',
+  })
+  body!: string;
+  @ApiProperty({
+    type: String,
+    nullable: true,
+    format: 'uuid',
+    description: 'The top-level comment it answers; null for a top-level one',
+  })
+  replyTo!: string | null;
+  @ApiProperty({
+    description: 'Deleted, but kept (without its text) while it has replies',
+  })
+  deleted!: boolean;
   @ApiProperty({ format: 'date-time' }) createdAt!: string;
   @ApiProperty({ type: String, nullable: true, format: 'date-time' })
   editedAt!: string | null;