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/schema-markup/SKILL.md

@@ -176,3 +176,4 @@ You can combine multiple schema types on one page using `@graph`:
 - **seo-audit**: For overall SEO including schema review
 - **ai-seo**: For AI search optimization (schema helps AI understand content)
 - **programmatic-seo**: For templated schema at scale
+- **site-architecture**: For breadcrumb structure and navigation schema planning