Migrate build and deployment pipeline from Cloudflare Pages to GitHub Pages.
Adds production-ready deploy workflow in deploy-github-pages.yml with proper
artifact handling. Removes Cloudflare-specific tooling: wrangler config, _headers,
_redirects, and CSP hash injection scripts (no longer needed with GitHub Pages
static hosting). Updates package.json build scripts and all project documentation
to reflect new deployment target and simplified architecture.