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>

apps/api/schema.gql

@@ -196,11 +196,19 @@ type CommandResult {
 
 type Comment {
   author: UserRef!
+
+  """Empty on a deleted comment kept for its replies"""
   body: String!
   createdAt: String!
+
+  """Deleted, but kept (without its text) while it has replies"""
+  deleted: Boolean!
   editedAt: String
   id: ID!
   issueId: ID!
+
+  """The top-level comment it answers; null for a top-level one"""
+  replyTo: ID
   version: Int!
 }
 
@@ -602,7 +610,11 @@ type Me {
 type Mutation {
   """Add a step to a ticket's checklist"""
   addChecklistItem(input: AddChecklistItemInput!): CommandResult!
-  addComment(body: String!, issueId: ID!): CommandResult!
+
+  """
+  Comment on a ticket, or answer a comment (replyTo; one level of replies)
+  """
+  addComment(body: String!, issueId: ID!, replyTo: ID): CommandResult!
 
   """
   Add a repository with its own secret (the first turns the integration on)
@@ -776,6 +788,7 @@ enum NotificationKind {
   MENTIONED
   QA_FAILED
   REMINDER
+  REPLIED
   STATUS_CHANGED
   WATCHED
 }