Add unit tests for remark plugin and nav builder

[braboj]

Summary

Zero automated tests exist in the project. The two pieces of custom logic — the remark link-rewriting plugin and the navigation builder — have no test coverage.

Deliverables

• Choose a test runner (vitest recommended — Astro ecosystem)
• Add tests for remark-rewrite-links plugin (link rewriting, edge cases, no-match)
• Add tests for nav/sidebar builder logic
• Add test script to package.json
• Add test stage to CI workflow (build.yml)
• Document in PLAYBOOK.md

Found by

360-degree analysis (Quality category), 2026-04-29.