feat: update clean-code skill and stabilize registry (#69)

- Updated clean-code skill with Robert C. Martin's Clean Code principles
- Refined content: naming, functions, comments, error handling, class design
- Fixed invalid heading format (## ## When to Use -> ## When to Use)
- Stabilized registry: use SOURCE_DATE_EPOCH for deterministic CI builds
- Improved catalog sorting for cross-environment consistency
- Regenerated all catalog and index files

Co-authored-by: jackjin1997 <jackjin1997@users.noreply.github.com>

data/bundles.json

@@ -1,5 +1,5 @@
 {
-  "generatedAt": "2026-02-06T08:53:44.091Z",
+  "generatedAt": "2026-02-08T10:01:43.886Z",
   "bundles": {
     "core-dev": {
       "description": "Core development skills across languages, frameworks, and backend/frontend fundamentals.",