Merge rubytest-cli back into rubytest with bin/rubytest executable.
Replace custom Indexer system with standard gemspec, Travis CI with
GitHub Actions, and Assembly with Rakefile. Remove Ruby 1.9 compat code,
simplify version handling. Fix typos, update URLs to HTTPS, move site
from gh-pages to docs/, and clean up obsolete files.

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