Find each node's management interface, and lift the agent's memlock limit

The agent's defaults said eth0 before normalize looked for the interface
holding the management address, so it never looked: a node on ens3 or
bond0 had NCCL and Gloo bootstrap over an interface it does not have. It
is now found, with eth0 only as the last resort.

The agent unit sets LimitMEMLOCK=infinity. RDMA pins what it registers,
and under systemd's 8 MB default an engine without the capability that
lets root ignore the limit fails its first collective over InfiniBand.

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

deploy/bootstrap/onboard-node.sh

@@ -226,6 +226,10 @@ ExecStart=/usr/local/bin/llmgw agent --config /etc/llmgw/agent.yaml
 KillMode=process
 Restart=on-failure
 RestartSec=2s
+# RDMA pins the buffers it registers, and NCCL registers a great deal: with
+# systemd's default 8 MB lock limit, an engine that drops the capability that
+# lets root ignore it fails its first collective over InfiniBand.
+LimitMEMLOCK=infinity
 StateDirectory=llmgw-agent
 
 [Install]