build(deploy): Jekyll 4 + minima 2.5 via GitHub Actions Pages

[davidbudzynski]

Moves deployment off the legacy branch-based Pages build onto a custom Actions workflow, unpinning us from the github-pages gem set.

What changes

• Gemfile: drops github-pages (Jekyll 3.10 / minima 2.5.1) for jekyll ~> 4.4.1, minima ~> 2.5.2, explicit feed/sitemap/seo-tag/redirect-from. Gemfile.lock regenerated (40 gems, was ~100).
• New .github/workflows/pages.yml: build (setup-ruby 3.4 + bundler cache, configure-pages, JEKYLL_ENV=production jekyll build --strict-front-matter --future, upload-pages-artifact) + deploy (deploy-pages, github-pages env, pages/id-token write, concurrency group).
• .ruby-version pins 3.4 to match CI.
• _config.yml + test.yml: comment-only updates (skin note corrected: minima 2.5.x ignores skin; htmlproofer note no longer references the github-pages set).

Verification (local, Ruby 4.0.6)

• bundle install green
• JEKYLL_ENV=production bundle exec jekyll build --trace --strict-front-matter --future green (only upstream Dart Sass deprecation warnings from minima _base.scss)
• ruby -Itests tests/test_site.rb: 33 runs, 598 assertions, 0 failures
• htmlproofer internal: success on 25 files
• Spot-checked: legacy /general/... redirect pages, feed.xml, sitemap.xml, search.json, tags, utterances embed

Manual step after merge

• Settings -> Pages -> Source: switch to GitHub Actions, then confirm the deploy job goes green and the live URL updates.