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/presenters/src/lib/ui/notifications-bell.ts

@@ -27,6 +27,7 @@ const ICON: Record<NotificationKind, string> = {
   [NotificationKind.QA_FAILED]: 'exclamation',
   [NotificationKind.COMMENTED]: 'chat',
   [NotificationKind.MENTIONED]: 'chat',
+  [NotificationKind.REPLIED]: 'chat',
   [NotificationKind.WATCHED]: 'focus',
   [NotificationKind.ADDED_TO_PROJECT]: 'briefcase',
   [NotificationKind.REMINDER]: 'calendar',