Merge feature/on-page-seo into development

Thorough on-page SEO: SeoHelper + shared/_seo emit title, description, canonical, Open Graph, Twitter Card, and JSON-LD on every page; per-page metadata across the public pages; robots.txt crawl rules; and a dynamic /sitemap.xml of static pages plus public repositories and profiles. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>