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/sagas/commenters-follow.saga.spec.ts

@@ -11,6 +11,7 @@ describe('CommentersFollowSaga', () => {
       projectId: 'p-1',
       authorId: 'ana',
       body: 'Lo miro',
+      replyTo: null,
       deleted: false,
       createdAt: '2026-10-08T10:00:00.000Z',
       editedAt: null,