chore: remove Jekyll files after confirming the Astro cutover

[devFancy]

Background

The Astro cutover is done but the Jekyll files are still in the repository. They were kept on purpose so there was a way back.

Goal

Delete the Jekyll files and make src/content/posts/ the source of truth.

Plan

1. Tag the commit just before the cutover as jekyll-final.
2. Delete _posts/, _config.yml, _includes/, _layouts/, _sass/, the Gemfile family, css/, js/, index.html, page/, search.json, feed.xml, sitemap.xml, the duplicate favicon.ico and robots.txt at the root, and scripts/convert-posts.mjs.
3. Remove the Jekyll entries from .gitignore.

Verify

• The build still produces 325 pages.
• The site verification files and ads.txt are untouched.

Notes

• Deleting the converter closes the path that regenerates posts from _posts/. It stays in the tag and in the history.
• Repository size does not go down.