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/pages/issue.page.scss

@@ -531,3 +531,32 @@ a.ts-key {
   border-radius: 0.5rem;
   background: color-mix(in srgb, var(--ts-c-red, #dc2626) 12%, transparent);
 }
+
+// One level of replies under each comment.
+.ts-replies {
+  display: grid;
+  gap: 0.75rem;
+  margin: -0.25rem 0 0.5rem 2.75rem;
+  padding-left: 0.875rem;
+  border-left: 2px solid var(--ts-line);
+
+  &:empty {
+    display: none;
+  }
+}
+
+.ts-comment__gone {
+  margin: 0;
+  font-style: italic;
+}
+
+.ts-reply__open {
+  justify-self: start;
+  font-size: 0.8125rem;
+}
+
+.ts-reply__actions {
+  display: flex;
+  gap: 0.75rem;
+  align-items: center;
+}