♻️ 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/systematic-debugging/test-academic.md

@@ -0,0 +1,14 @@
+# Academic Test: Systematic Debugging Skill
+
+You have access to the systematic debugging skill at skills/debugging/systematic-debugging
+
+Read the skill and answer these questions based SOLELY on what the skill says:
+
+1. What are the four phases of systematic debugging?
+2. What must you do BEFORE attempting any fix?
+3. In Phase 3, what should you do if your first hypothesis doesn't work?
+4. What does the skill say about fixing multiple things at once?
+5. What should you do if you don't fully understand the issue?
+6. Is it ever acceptable to skip the process for simple bugs?
+
+Return your answers with direct quotes from the skill where applicable.