Add note on using defaultValue with a json code field

Author: andrewmumblebee

.claude/skills/build-plugin/references/ui.md

@@ -260,6 +260,12 @@ Choosing between them: use **`radio`** when each option reads best on its own li
 { "type": "code", "name": "body", "label": "Request Body", "language": "json" }
 ```
 
+> ⚠️ When `language` is `json`, `defaultValue` must be a JSON object, not a string — the editor calls `JSON.stringify` on it. A string `defaultValue` renders as a quoted, escaped blob instead of formatted JSON.
+>
+> ```json
+> { "type": "code", "name": "body", "language": "json", "defaultValue": { "key": "value" } }
+> ```
+
 **`script`** — inline JavaScript editor:
 
 ```json