♻️ refactor(skills): update playbook.py and tests for thirdparty/ layout

- install_skills_action: collect skills from both codex/skills/ (own) and
  codex/skills/thirdparty/ (thirdparty); mode=all installs both, mode=list
  searches both; log [thirdparty] tag for third-party installs
- Update test_thirdparty_skills_pipeline: fix source_list path assertion
- Update SKILLS.md: document thirdparty/ subdirectory structure

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

SKILLS.md

@@ -28,11 +28,15 @@ skills = true
 
 ```txt
 codex/skills/
-  <skill-name>/
+  <skill-name>/         # 本仓库自维护
     SKILL.md
     references/        # 可选：拆分参考文档
     templates/         # 可选：模板
     scripts/           # 可选：脚本/命令封装
+  thirdparty/          # 第三方同步（不可手动修改）
+    <skill-name>/
+      SKILL.md
+    .sources/          # 第三方来源清单
 ```
 
 最终安装到本机后，对应路径为：
@@ -167,7 +171,7 @@ python <deploy_root>/scripts/playbook.py -config playbook.toml
 
 ## 9. Third-party Skills
 
-来源：`codex/skills/.sources/`（第三方来源清单目录）。
+来源：`codex/skills/thirdparty/.sources/`（第三方来源清单目录）。
 
 - `superpowers.list`
 - `ui-ux-pro-max.list`