Unify GitHub Pages site

[PatrickJS]

Summary

• Add the shared Async Pages builder and deploy .async/pages from generated @async/pipeline workflows.
• Move the Pages job from the old docs artifact path to docs.site.
• Update the workflow ownership test to assert the standardized Pages artifact and synced docs.site script.

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 the db shared Pages layout

[github-actions]

Preview package

Preview for PR head 19418b59b7a6c9203d732aa4d2a50be97b23c791 (built from its merge with main), published to GitHub Packages as @async/db.

Latest successful build for this PR:

pnpm add @async/db@pr-27

Exact commit build:

pnpm add @async/db@0.0.0-pr.27.sha.19418b59b7a6c9203d732aa4d2a50be97b23c791

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