test(ci): add release workflow validation tests

Add test_release_workflow.py with 5 tests that validate the release
workflow configuration:

- YAML syntax validity and presence of jobs
- Author attribution token in changelog format
- No broken docs/guides/* links referencing missing files
- Old "Thank you" contributor format is absent
- README.md reference exists for installation instructions

Tests use static YAML parsing and regex checks against the workflow
file content, requiring no network access or CI execution.