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/web/tracker/infrastructure/src/lib/adapters/graphql-collaboration.gateways.ts

@@ -42,9 +42,13 @@ export class GraphqlCommentsGateway extends CommentsGateway {
       .pipe(map((d) => d.issue.comments.items.map(toComment)));
   }
 
-  add(issueId: string, body: string): Observable<CommandResult> {
+  add(
+    issueId: string,
+    body: string,
+    replyTo: string | null = null,
+  ): Observable<CommandResult> {
     return this.gql
-      .mutate(AddCommentDocument, { issueId, body })
+      .mutate(AddCommentDocument, { issueId, body, replyTo })
       .pipe(map((d) => toCommandResult(d.addComment)));
   }