Replace custom Indexer system with standard gemspec, Travis CI with
GitHub Actions, and Assembly/detroit with Rakefile. Update minitest
adapter for minitest 5+ and testunit adapter for modern test-unit gem.
Simplify version.rb. Fix typos, update URLs to HTTPS, add LICENSE.txt,
move site from gh-pages to docs/, and clean up obsolete files.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>