docs: remove common-cache references from documentation and tooling

Remove all remaining references to common-cache from project documentation,
test workflows, and build tooling after action deletion.

Updated:
- CLAUDE.md: Remove from action catalog (28 → 27 actions)
- README.md: Regenerate catalog without common-cache
- SECURITY.md: Update caching optimization notes
- Test workflows: Remove common-cache test references
- spec_helper.sh: Remove common-cache test helpers
- generate_listing.cjs: Remove from category/language mappings
- update-validators.py: Remove custom validator entry

SECURITY.md

@@ -231,7 +231,7 @@ When security issues are fixed:
 - Replaced custom Bun installation with official action
 - Replaced custom Trivy installation with official action
 - Added secret masking to 7 critical actions (including docker-publish)
-- Optimized file hashing in common-cache
+- Migrated from custom common-cache to official actions/cache
 - Status: ✅ Complete
 
 ### Phase 3: Documentation & Policy (2024)