Add the Kinhluan Fieldbook

[kinhluan]

Summary

• Add a source-driven Astro fieldbook for browsing the repository's skills.
• Organize the experience around Ngũ Sự, five active pursuits, and five learning paths.
• Generate the library and all skill detail pages from the canonical skills.json and skills/*/SKILL.md sources.
• Add GitHub Pages deployment and pull-request validation workflows.

Why

The repository has strong skill content but no public, browsable experience that explains what the collection is for or how to start using it. The fieldbook makes that content discoverable without creating a second copy to maintain.

Ngũ Sự acts as the cross-cutting strategic lens, while pursuits, paths, and the searchable library support different ways of exploring the same source material.

Impact

• Adds a new static site under site/ with 71 generated pages.
• Publishes to https://kinhluan.github.io/skills/ after merge and GitHub Pages setup.
• Automatically reflects future canonical skill updates in generated pages.
• Requires repository maintainers to select GitHub Actions as the Pages source once.
• Does not change skill package or runtime behavior.

Stacked dependency

This PR is stacked on #19 and targets agent/audit-and-harden-skills. Review and merge #19 first, then retarget this PR to main if GitHub does not adjust the base automatically.

Validation

• make site-check — 5 site tests passed; Astro built 71 pages; 71 routes and internal links verified.
• make check — 15 repository tests passed; 64 skills validated; 64 deterministic packages verified.