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/issues.controller.ts

@@ -39,7 +39,7 @@ import {
   BulkUpdateIssuesDto,
   ChangeStatusDto,
   ChecklistItemBodyDto,
-  CommentBodyDto,
+  AddCommentDto,
   CreateIssueDto,
   IssueSearchQueryDto,
   LinkIssueDto,
@@ -529,18 +529,21 @@ export class IssuesController {
 
   @Post('issues/:issue/comments')
   @ISSUE
-  @ApiOperation({ summary: 'Comment on the ticket' })
+  @ApiOperation({
+    summary: 'Comment on the ticket, or answer a comment (replyTo)',
+  })
   @ApiCreatedResponse({ type: CommandResultDto })
   @ApiCommandErrors()
   async addComment(
     @CurrentUser() me: User,
     @Param('issue') issue: string,
-    @Body() body: CommentBodyDto,
+    @Body() body: AddCommentDto,
   ) {
     return this.comments.add(
       me.id,
       await this.refs.issueId(me.id, issue),
       body.body,
+      body.replyTo ?? null,
     );
   }