ci: add packaging quality gate

[kholdrex]

Summary

Adds a dedicated CI quality job for irt_ruby so lint and package verification run alongside the existing Ruby compatibility test matrix. The new packaging check builds the gem, installs the built artifact, and smoke-tests the installed gem from a temporary directory.

Changes

• Updates GitHub Actions to use actions/checkout@v4, quote Ruby matrix versions, and keep matrix failures visible across supported Rubies.
• Adds a CI quality job for RuboCop plus gem build/install verification.
• Updates contributor documentation and the changelog with the local quality and packaging commands.

Test plan

• RSpec suite passed.
• RuboCop passed.
• Default Rake quality gate passed.
• Gem build, install, and installed-gem smoke test passed.