♻️ 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>

codex/skills/thirdparty/.sources/andrej-karpathy-skills.list

@@ -0,0 +1 @@
+karpathy-guidelines

codex/skills/thirdparty/.sources/superpowers.list

@@ -0,0 +1,14 @@
+brainstorming
+dispatching-parallel-agents
+executing-plans
+finishing-a-development-branch
+receiving-code-review
+requesting-code-review
+subagent-driven-development
+systematic-debugging
+test-driven-development
+using-git-worktrees
+using-superpowers
+verification-before-completion
+writing-plans
+writing-skills

codex/skills/thirdparty/.sources/ui-ux-pro-max.list

@@ -0,0 +1 @@
+ui-ux-pro-max