feat: add SEO preflight rules (seo.* family)

[yasserstudio]

Summary

• 7 new preflight rules under the seo.* family — all default to info severity (non-gating, advisory only)
• seo.title-length — flags titles outside the 30-150 char optimal range for Google Shopping
• seo.title-brand — checks that the brand name appears in the product title
• seo.title-attributes — checks that color/size appear in the title when set on the product
• seo.description-length — flags descriptions outside the 500-5000 char range
• seo.title-equals-description — catches identical title and description (wasted SEO)
• seo.description-has-brand — checks brand is mentioned in the description
• seo.image-placeholder — detects placeholder/default image URLs
• Adds color and size to the ProductAttributes type

Users can promote any rule to warning or error in .gmcpreflightrc:

{ "rules": { "seo.title-length": "warning" } }

Test plan

• 21 new tests for all 7 rules (positive, negative, absent-field cases)
• Registry test updated to include seo family
• Existing CLI preflight + migrate tests pass with enriched fixtures
• Build, typecheck, lint all green

🤖 Generated with Claude Code

https://claude.ai/code/session_01W3y7L3ZFJnJPcN2ZhGXmXQ