Add Chinese documentation for custom skills development and reference guide

- Created a new document for custom skills development (`custom.md`) detailing the structure, creation, implementation, and best practices for developing custom CCW skills.
- Added an index document (`index.md`) summarizing all built-in skills, their categories, and usage examples.
- Introduced a reference guide (`reference.md`) providing a quick reference for all 33 built-in CCW skills, including triggers and purposes.

ccw/frontend/src/locales/zh/review-session.json

@@ -96,6 +96,11 @@
     "message": "尝试调整筛选条件或搜索查询。",
     "noFixProgress": "无修复进度数据"
   },
+  "notExecuted": {
+    "title": "审查尚未执行",
+    "message": "此审查会话已创建，但审查流程尚未启动。尚未生成任何发现结果。",
+    "hint": "💡 提示：请执行审查工作流以开始分析代码并生成发现结果。"
+  },
   "notFound": {
     "title": "未找到审查会话",
     "message": "无法找到请求的审查会话。"