chore: Add CI validation jobs

[marandaneto]

💡 Motivation and Context

Improve CI coverage so pull requests validate more than the RSpec matrix. This helps catch RuboCop issues and gem packaging problems before release while keeping workflow token permissions scoped to read-only repository contents.

💚 How did you test it?

• Parsed .github/workflows/unit-tests.yml with Ruby YAML loader (YAML OK).
• Not run locally beyond YAML validation; GitHub Actions workflow-only change.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran pnpm changeset to generate a changeset file

[greptile-apps]

_{Reviews (1): Last reviewed commit: "chore: add CI validation jobs" | Re-trigger Greptile}

[greptile-apps]

_{Reviews (2): Last reviewed commit: "chore: limit workflow token permissions" | Re-trigger Greptile}

[dustinbyrne]

required checks will need to be updated here as well