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/openapi.json

@@ -4406,7 +4406,7 @@
           "content": {
             "application/json": {
               "schema": {
-                "$ref": "#/components/schemas/CommentBodyDto"
+                "$ref": "#/components/schemas/AddCommentDto"
               }
             }
           }
@@ -4491,7 +4491,7 @@
             "pireon": []
           }
         ],
-        "summary": "Comment on the ticket",
+        "summary": "Comment on the ticket, or answer a comment (replyTo)",
         "tags": [
           "Issues"
         ]
@@ -11423,7 +11423,18 @@
             "$ref": "#/components/schemas/UserRefDto"
           },
           "body": {
-            "type": "string"
+            "type": "string",
+            "description": "Empty on a deleted comment kept for its replies"
+          },
+          "replyTo": {
+            "type": "string",
+            "nullable": true,
+            "format": "uuid",
+            "description": "The top-level comment it answers; null for a top-level one"
+          },
+          "deleted": {
+            "type": "boolean",
+            "description": "Deleted, but kept (without its text) while it has replies"
           },
           "createdAt": {
             "type": "string",
@@ -11443,6 +11454,8 @@
           "issueId",
           "author",
           "body",
+          "replyTo",
+          "deleted",
           "createdAt",
           "editedAt",
           "version"
@@ -11466,13 +11479,18 @@
           "total"
         ]
       },
-      "CommentBodyDto": {
+      "AddCommentDto": {
         "type": "object",
         "properties": {
           "body": {
             "type": "string",
             "example": "Reproducido en Safari 17.",
             "maxLength": 20000
+          },
+          "replyTo": {
+            "type": "string",
+            "format": "uuid",
+            "description": "Comment it answers (one level: answering a reply joins its thread)"
           }
         },
         "required": [
@@ -11497,6 +11515,19 @@
           "file"
         ]
       },
+      "CommentBodyDto": {
+        "type": "object",
+        "properties": {
+          "body": {
+            "type": "string",
+            "example": "Reproducido en Safari 17.",
+            "maxLength": 20000
+          }
+        },
+        "required": [
+          "body"
+        ]
+      },
       "ReleaseStatus": {
         "type": "string",
         "enum": [
@@ -11929,6 +11960,7 @@
           "QA_FAILED",
           "COMMENTED",
           "MENTIONED",
+          "REPLIED",
           "WATCHED",
           "ADDED_TO_PROJECT",
           "REMINDER"