Release v2.2.0: AMD GPU Monitoring

Complete AMD GPU monitoring implementation:
- GPU utilization, memory, temperature, power, clocks, fan
- Multi-GPU support with automatic device enumeration
- Pure Go implementation (no CGO)
- Supported: RX 7900, 6900, 6800, 6700, 6600, Vega, RX 580/570
- Configuration: metrics.enabled.gpu

See CHANGELOG.md for full release notes.