Hotfix: restore Jekyll Pages rendering (remove .nojekyll)

[viyatb]

Hotfix: restore GitHub Pages Jekyll rendering

This fixes the live site rendering issue where front matter and Liquid includes were shown as raw text.

Root cause

A compatibility workaround added .nojekyll, which disables Jekyll processing on GitHub Pages branch-based publishing.

Fixes in this PR

1. Remove .nojekyll from the repository.
2. Remove _next/.keep compatibility stub.
3. Update .github/workflows/main.yml Static Integrity checks to validate the current Jekyll source model instead of legacy .nojekyll + _next artifacts.
4. Keep link validation for both homepage and vendored online-passive-scanner pages, with runtime-placeholder exceptions.

Local verification

Ran the updated integrity script locally:

• Static site integrity checks passed.

After merge, GitHub Pages should process Jekyll templates/includes again and render the normal site.