♻️ 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>

.gitea/ci/thirdparty_skills.json

@@ -6,7 +6,7 @@
       "upstream_ref": "main",
       "snapshot_dir": "superpowers",
       "sync_mode": "copy_skill_dirs",
-      "source_list": "codex/skills/.sources/superpowers.list",
+      "source_list": "codex/skills/thirdparty/.sources/superpowers.list",
       "skills_subdir": "skills",
       "remove_paths": ["skills/ui-ux-pro-max"]
     },
@@ -16,7 +16,7 @@
       "upstream_ref": "main",
       "snapshot_dir": "ui-ux-pro-max",
       "sync_mode": "render_codex_skill",
-      "source_list": "codex/skills/.sources/ui-ux-pro-max.list",
+      "source_list": "codex/skills/thirdparty/.sources/ui-ux-pro-max.list",
       "output_name": "ui-ux-pro-max",
       "platform_config": "src/ui-ux-pro-max/templates/platforms/codex.json",
       "template_root": "src/ui-ux-pro-max/templates",
@@ -29,7 +29,7 @@
       "upstream_ref": "main",
       "snapshot_dir": "andrej-karpathy-skills",
       "sync_mode": "copy_skill_dirs",
-      "source_list": "codex/skills/.sources/andrej-karpathy-skills.list",
+      "source_list": "codex/skills/thirdparty/.sources/andrej-karpathy-skills.list",
       "skills_subdir": "skills"
     }
   ]