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>

scripts/build-catalog.js

@@ -318,9 +318,9 @@ function buildCatalog() {
   }
 
   const catalog = {
-    generatedAt: new Date().toISOString(),
+    generatedAt: process.env.SOURCE_DATE_EPOCH ? new Date(process.env.SOURCE_DATE_EPOCH * 1000).toISOString() : new Date().toISOString(),
     total: catalogSkills.length,
-    skills: catalogSkills.sort((a, b) => a.id.localeCompare(b.id)),
+    skills: catalogSkills.sort((a, b) => (a.id < b.id ? -1 : a.id > b.id ? 1 : 0)),
   };
 
   const aliases = buildAliases(catalog.skills);