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/requests.dto.ts

@@ -955,6 +955,17 @@ export class CommentBodyDto {
   body!: string;
 }
 
+export class AddCommentDto extends CommentBodyDto {
+  @ApiPropertyOptional({
+    format: 'uuid',
+    description:
+      'Comment it answers (one level: answering a reply joins its thread)',
+  })
+  @IsOptional()
+  @IsUUID()
+  replyTo?: string;
+}
+
 // ---- Releases ---------------------------------------------------------------
 
 export class CreateReleaseDto {