Release v0.7.0

Major release with new command-based CLI structure

Features:
- New command-based CLI using cobra framework
- Commands: sync, list, manage, release, showcase, test
- Comprehensive help system with examples
- AI-powered release notes generation
- Updated default config path to ~/.config/gitsyncer/config

Improvements:
- Better organization of functionality
- Consistent flag naming across commands
- Enhanced error messages and user experience
- Backward compatibility for old flags (with deprecation warnings)

Breaking Changes:
- Default config location changed from ~/.gitsyncer.json to ~/.config/gitsyncer/config
- Old flag syntax is deprecated (but still works)

See README.md for updated usage instructions.