feat: Implement comprehensive cross-platform command execution system

- Add core cross-platform command execution rule at formal system level
- Create utils/cross_platform/ system with automatic OS detection
- Implement CrossPlatformAgent base class for all agents
- Add OS-appropriate command generation (Windows/Linux/macOS)
- Fix utils.validation module import issues with proper __init__.py
- Resolve TestConfig import error in UI tests
- Enable systematic OS-aware development preventing hardcoded commands
- Establish foundation for 100% cross-platform compatibility

Author: gitwalter