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/domain/src/lib/ports/gateways.ts

@@ -230,7 +230,11 @@ export abstract class SavedFiltersGateway {
 
 export abstract class CommentsGateway {
   abstract list(issueId: string): Observable<Comment[]>;
-  abstract add(issueId: string, body: string): Observable<CommandResult>;
+  abstract add(
+    issueId: string,
+    body: string,
+    replyTo?: string | null,
+  ): Observable<CommandResult>;
   abstract edit(commentId: string, body: string): Observable<CommandResult>;
   abstract remove(commentId: string): Observable<CommandResult>;
 }