Same pattern as opencode-ruby's CI. Workflow runs test suite + gem
build + post-install smoke load on Ruby 3.2 / 3.3 / 3.4.
Note: the 'Verify gem loads after install' step depends on
opencode-ruby being resolvable. Once both gems are on rubygems.org
the runtime_dependency resolves automatically; until then, contributors
who want to run the full workflow locally need to either install
opencode-ruby first or rely on the existing 'bundle exec rake test'
step (which uses the Gemfile-resolved dep, not the gem-spec-resolved
one).
