feat(skills): add frontend-design from anthropic-agent-skills

Source: anthropic-agent-skills plugin cache → skills-external/frontend-design/
Auto-install via install-plugins.sh step 8b, auto-update via update-all.sh
step 7.1, symlinked by link.sh (refactored to loop over EXTERNAL_SKILLS).
Added to profiles: design, web, web-full, full.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/profiles/web.profile

@@ -32,8 +32,9 @@ refactor                          personal
 # Validation companion (basic W3C/a11y check during build)
 validate                          personal
 
-# External: Emil Kowalski's polish philosophy
+# External: design skills
 emil-design-eng                   external
+frontend-design                   external
 
 # Plugin: UI/UX intelligence (auto-toggle)
 ui-ux-pro-max                     plugin@ui-ux-pro-max-skill