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/application/src/lib/event-handlers/activity-summary.ts

@@ -200,7 +200,9 @@ export function describeEvent(
     return `${actor} quitó «${event.text}» de la lista de ${issueKey}`;
 
   if (event instanceof CommentAddedEvent)
-    return `${actor} comentó en ${issueKey}`;
+    return event.comment.replyTo
+      ? `${actor} respondió a un comentario en ${issueKey}`
+      : `${actor} comentó en ${issueKey}`;
   if (event instanceof CommentEditedEvent)
     return `${actor} editó un comentario en ${issueKey}`;
   if (event instanceof CommentDeletedEvent)