Unify GitHub Pages site

[PatrickJS]

Summary

• Add the shared Async Pages builder and dark docs shell.
• Switch the generated Pages workflow to build .async/pages through docs.site.
• Sync pipeline package scripts so docs.site is available via pnpm run pipeline:task:docs.site.

Verification

• pnpm run pipeline:task:docs.site
• pnpm run pipeline:sync:check && pnpm run pipeline:github:check
• pnpm run release:check
• local Chrome/Playwright visual smoke covered representative shared Pages layout

[github-actions]

Preview package

Preview for PR head dc66d305365e747de2035c2c01179c5a065519a3 (built from its merge with main), published to GitHub Packages as @async/auto-git.

Latest successful build for this PR:

pnpm add @async/auto-git@pr-4

Exact commit build:

pnpm add @async/auto-git@0.0.0-pr.4.sha.dc66d305365e747de2035c2c01179c5a065519a3

Requires GitHub Packages auth and @async:registry=https://npm.pkg.github.com in your npm config.