feat: add Cloudflare Pages project website

[wimi321]

Summary

Adds a simple, professional Astro static website for goagent, ready for Cloudflare Pages deployment at goagent.top.

What changed

• Adds website/ as an independent Astro static project.
• Adds pages for home, download, docs, privacy, FAQ, and changelog.
• Adds Cloudflare Pages headers, redirects, robots.txt, logo, and deployment guide.
• Adds root scripts for website install/build/dev/check.
• Adds scripts/check_website.mjs to guard required pages, goagent.top domain usage, GitHub Releases links, privacy language, and no large installer files in website public assets.
• Adds pnpm-workspace.yaml so root pnpm install installs website dependencies too.

Deployment

Cloudflare Pages settings:

• Root directory: website
• Build command: pnpm build
• Output directory: dist
• Custom domains: goagent.top, www.goagent.top

Verification

• pnpm install
• pnpm website:build
• pnpm check:website
• local HTTP smoke on /, /download, /docs, /privacy, /faq, /changelog
• pnpm test
• pnpm typecheck