ci: remove banned Jekyll workflow (per estate language policy) (#31)

## Summary

`jekyll-gh-pages.yml` is a banned-language workflow per the 2026-05-25
estate language policy ([[feedback_no_jekyll_use_casket_ssg]]). It fails
on every push and red-mains this repo.

## Fix

Delete the workflow file. For Pages publishing, use `casket-pages.yml`
(canonical casket-ssg-based template in hyperpolymath/standards).
Tracked under hyperpolymath/standards#306.

## Estate-wide context

One of 12 repos affected. Caught by the 2026-05-30 estate CI/CD audit.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: hyperpolymath