Read NVLink and GPU-to-adapter distance from nvidia-smi topo

The release binary is built without cgo, so without NVML, and NVML was
the only source of NVLink peers: on every NVLink machine the portable
agent reported none, and tensor-parallel groups were chosen by GPU index.
Discovery now reads `nvidia-smi topo -m` after either path, for the
NVLink peers (kept from NVML where it gave them), each card's NUMA node,
and how far each network adapter is from it - PIX, PXB, PHB, NODE or
SYS - which travels to the control plane as nic_paths for placement to
use. Both the NICn-with-legend layout of newer drivers and the
adapter-named columns of older ones are read.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

api/proto/llmgw/v1/agent.proto

@@ -207,6 +207,9 @@ message GPU {
   // Zero when the driver is too old to report it, which is not the same as
   // idle: the metric is simply absent rather than wrong.
   int32 utilization_percent = 15;
+  // nic_paths is how far each network adapter is from this card, by HCA
+  // name: PIX, PXB, PHB, NODE or SYS, from nvidia-smi topo.
+  map<string, string> nic_paths = 16;
 }
 
 message FabricAttachment {