add PR conventional commits check

[dangrondahl]

Add a CI workflow to validate PR titles follow the Conventional Commits format,
mirroring the setup in `kosli-dev/terraform-provider-kosli`.

Tasks

• Port `.github/workflows/pr-quality.yml` from `terraform-provider-kosli`
• Validate allowed types: `feat`, `fix`, `docs`, `style`, `refactor`, `perf`, `test`, `build`, `ci`, `chore`, `revert`
• Enforce lowercase subject (no uppercase first letter)
• Scope should be optional

References

• terraform-provider-kosli/pr-quality.yml
• https://www.conventionalcommits.org/