Add K3s agent setup with NVMe-backed persistent storage

Bind-mount K3s agent data, node identity, and kubelet dirs from
NVMe so container image cache and node registration survive reboots
on the diskless netboot nodes. Includes K3s binary download, agent
systemd service, DHCP hostname resolution, and open-iscsi for
Longhorn iSCSI support.

files/k3s-join.service

@@ -0,0 +1,26 @@
+[Unit]
+Description=K3s Agent Join Service
+Documentation=file:///usr/local/bin/k3s-join
+
+# Run after network and storage are ready
+After=network-online.target setup-node-storage.service
+Wants=network-online.target
+Requires=containerd.service
+
+[Service]
+Type=exec
+ExecStart=/usr/local/bin/k3s-join
+Restart=on-failure
+RestartSec=30
+
+# Environment
+Environment="PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
+
+# Hardening
+LimitNOFILE=1048576
+LimitNPROC=infinity
+LimitCORE=infinity
+TasksMax=infinity
+
+[Install]
+WantedBy=multi-user.target