♻️ refactor(skills): separate thirdparty skills into thirdparty/ subdirectory

- Move all third-party skills from codex/skills/ to codex/skills/thirdparty/
- Move .sources/ to codex/skills/thirdparty/.sources/
- Delete systematic-debugging/CREATION-LOG.md (internal dev note, not skill content)
- Update sync_thirdparty_skills.sh paths and conflict detection
- Update thirdparty_skills.json source_list paths
- Update install_skills_action to collect from both own and thirdparty dirs,
  logging [thirdparty] tag for third-party installs
- Update SKILLS.md directory structure docs
- Update tests to reflect new paths

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

rulesets/cpp/index.md

@@ -43,5 +43,4 @@
 
 ## 与开发规范的关系
 
-- 本仓库内：`docs/cpp/` 与 `docs/common/`
-- 目标项目 subtree：`docs/standards/playbook/docs/cpp/` 与 `docs/standards/playbook/docs/common/`
+- 规范文档：`docs/cpp/` 与 `docs/common/`