feat(mcp): add cross-platform Windows support to filesystem example

Implement automatic OS detection to support Windows, Linux, and macOS
without code changes. Windows requires cmd.exe wrapper for npx batch
files, while Unix systems execute npx directly.

Changes:
- Add isWindows() method with conditional ServerParameters creation
- Support both programmatic and JSON configuration approaches
- Use @ConditionalOnMissingBean to avoid bean conflicts
- Add Windows and Linux JSON configuration files
- Update README with cross-platform quick start and troubleshooting
- Use relative paths for better portability

Author: markpollack