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

generate_listing.cjs

@@ -47,7 +47,6 @@ const CATEGORIES = {
   'sync-labels': 'Repository',
   stale: 'Repository',
   'compress-images': 'Repository',
-  'common-cache': 'Repository',
   'codeql-analysis': 'Repository',
 
   // Validation
@@ -85,7 +84,6 @@ const LANGUAGE_SUPPORT = {
   'release-monthly': ['GitHub Actions'],
   stale: ['GitHub Actions'],
   'compress-images': ['Images', 'PNG', 'JPEG'],
-  'common-cache': ['Caching'],
 };
 
 // Icon mapping for GitHub branding