fix(scenes): remove manual JoinRoom prefab (Cocos cache 引发 Maximum call stack 递归)

funplay duplicate_prefab 复刻的 manual 副本在双击打开时触发
'Maximum call stack size exceeded' (Prefab._instantiate 递归). 根因是 Cocos 资产
数据库 cache 状态差异, 与 prefab 数据无关. 脚本生成版 JoinRoom_Layer.prefab
(commit 1daa22d) 正常, validate 0 missing UUID.

结论: duplicate_prefab 复刻路径在复杂 prefab 上不可靠. 正确路径:
直接 Editor UI 拖节点 + funplay MCP 工具调用 (不依赖 cache).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

cocoscreator_projects/YouleNexus/assets/framework/ui/prefabs/_playbook_demo/JoinRoom_Layer_manual.prefab.meta

@@ -1,13 +0,0 @@
-{
-  "ver": "1.1.50",
-  "importer": "prefab",
-  "imported": true,
-  "uuid": "90017110-d0aa-4627-9a53-b306d5d73a7d",
-  "files": [
-    ".json"
-  ],
-  "subMetas": {},
-  "userData": {
-    "syncNodeName": "JoinRoom_Layer_manual"
-  }
-}