ci: remove banned Jekyll workflow (per estate language policy) (#21)

Jekyll is banned estate-wide per [[feedback_no_jekyll_use_casket_ssg]]
(banned 2026-05-25). `.github/workflows/jekyll-gh-pages.yml` was
failing on every push (Pages not enabled + banned-language policy red)
and contributing to red main on this repo.

For repos that genuinely need GitHub Pages publishing, use the canonical
casket-ssg-based workflow (`casket-pages.yml` from hyperpolymath/standards).
Tracked under hyperpolymath/standards#306.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath