ci: fix hypatia-scan working-directory + retire banned Jekyll workflow

- hypatia-scan.yml: the build step used working-directory: ${{ env.HOME }}/hypatia,
  but env.HOME is empty in GitHub's expression context, so it resolved to
  /hypatia (nonexistent) and the step failed to start. The clone and run steps
  used runtime $HOME. Align all three to $RUNNER_TEMP / ${{ runner.temp }},
  which is valid in BOTH the shell and expression contexts.
- Remove jekyll-gh-pages.yml: Jekyll is banned estate-wide (use casket-ssg).
  aLib is not a Jekyll site (no _config.yml/Gemfile, ships .nojekyll, docs are
  .adoc), so the workflow was vestigial — retired by deletion. Clears Hypatia
  jekyll_gh_pages_workflow_detected.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KPG9mEQXFyA3k7NWAzMNMr

Author: claude