infra: restore validation scripts

[marc0olo]

Context

Validation scripts from restructuring-attempt-1 need to be restored and updated for the current structure.

Scripts to restore

• scripts/validate-frontmatter.mjs — checks all pages have required frontmatter fields (title + description)

Steps

1. Check out restructuring-attempt-1 to review the original script
2. Adapt for current directory layout (docs/ not src/content/docs/)
3. Update frontmatter schema check to match current schema (title + description required)
4. Add to package.json as npm run validate
5. Wire into CI (see infra: restore sync scripts #115)

[marc0olo]

Already implemented:

• Frontmatter validation (title + description required) lives in scripts/validate.js (checkFrontmatter)
• Available as npm run validate
• Wired into CI via .github/workflows/validate.yml — runs on every PR against changed docs/**/*.md files and fails the workflow on errors