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/models/enums.ts

@@ -53,6 +53,7 @@ export enum NotificationKind {
   QA_FAILED = 'QA_FAILED',
   COMMENTED = 'COMMENTED',
   MENTIONED = 'MENTIONED',
+  REPLIED = 'REPLIED',
   WATCHED = 'WATCHED',
   ADDED_TO_PROJECT = 'ADDED_TO_PROJECT',
   REMINDER = 'REMINDER',