AMD ROCm GPU support

[pmady]

Currently the server only supports NVIDIA GPUs via NVML. Adding AMD ROCm support would make gpu-mcp-server vendor-neutral.

Requirements:

• Implement a new ROCmCollector that satisfies the gpu.Collector interface
• Use rocm-smi CLI or AMD ROCm SMI library for GPU metrics
• Auto-detect GPU vendor at startup and choose the right collector
• Maintain parity with NVML metrics where possible (utilization, memory, temperature, power)
• Add build tag //go:build cgo && linux (ROCm is Linux-only)

Challenges:

• ROCm has no official Go bindings — may need to shell out to rocm-smi --json or use CGO with librocm_smi64.so
• Some metrics may not have 1:1 equivalents between NVML and ROCm

Difficulty: Hard — requires AMD GPU hardware for testing