Create local GitHub Actions runner with tests

[thorwhalen]

Add complete test infrastructure for the GitHub Actions local runner:

• tests/test_local_cli.py: 40 unit tests covering all functions

  • Test dependency checking (act, Docker)
  • Test command construction and execution
  • Test CLI argument parsing
  • Test error handling and edge cases

• tests/test_local_cli_integration.py: 13 integration tests

  • Test with actual workflow files
  • Test job and matrix execution
  • Test command construction with all options
  • Test error scenarios

• tests/conftest.py: Pytest fixtures for test setup

  • Temporary workflow file fixture
  • Dependency mocking fixtures

• pytest.ini: Test configuration with markers

  • Markers for unit, integration, and optional tests
  • Standard test discovery patterns

• run_tests.sh: Convenient test runner script

  • Support for coverage reports
  • Pattern-based test selection
  • Verbose output option

• tests/README.md: Comprehensive test documentation

  • How to run tests
  • Test organization and structure
  • Coverage information
  • Troubleshooting guide

• LOCAL_CLI_USAGE.md: User documentation

  • Quick start guide
  • Installation instructions
  • Usage examples
  • Troubleshooting section

Test results: 51 passed, 2 skipped (require act/Docker installation)
Coverage: ~95% of local_cli.py code

All tests use mocking to avoid requiring act or Docker installation, making them runnable in any environment including CI systems.