Restore CI and RuboCop linting

[StantonMatt]

Summary

• add GitHub Actions CI for RSpec across the advertised Ruby 2.3-2.7 support range
• keep ruby-head in the test matrix as an allowed failure, matching the old Travis intent
• add a RuboCop job, update obsolete RuboCop cop names/options, and baseline existing lint debt in .rubocop_todo.yml

Notes

This follows up on the maintainer request in #40 to help update the CI/lint infra. I kept the PR scoped to infrastructure only; no runtime code changed.

I left Ruby 3.x expansion for a separate support-policy change. This PR mirrors the gemspec's current >= 2.3 support and the existing Travis-era range rather than raising the Ruby floor.

Verification

• ruby -ryaml -e 'YAML.load_file(".github/workflows/ci.yml"); puts "yaml ok"'
• bundle exec rspec
• bundle exec rubocop
• git diff --check
• review-fix-loop: clean, no actionable correctness issues

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[NullVoxPopuli]

ttyty

[StantonMatt]

Follow-up on the post-merge check display: the merged commit has a visible red Test Ruby head check because continue-on-error still leaves the individual matrix job red in the commit UI, even though the workflow conclusion is success.

I opened #42 to remove only that unsupported Ruby head matrix entry. I rechecked it just now: Ruby 2.3, 2.4, 2.5, 2.6, 2.7, RuboCop, and WIP are all green, and the PR is clean/mergeable.