feat: add site-architecture skill for website structure planning

Fills the gap between content strategy and SEO skills — covers page
hierarchy, navigation design, URL structure, and internal linking.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

skills/seo-audit/SKILL.md

@@ -406,6 +406,7 @@ Same format as above
 
 - **ai-seo**: For optimizing content for AI search engines (AEO, GEO, LLMO)
 - **programmatic-seo**: For building SEO pages at scale
+- **site-architecture**: For page hierarchy, navigation design, and URL structure
 - **schema-markup**: For implementing structured data
 - **page-cro**: For optimizing pages for conversion (not just ranking)
 - **analytics-tracking**: For measuring SEO performance