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

@@ -0,0 +1,78 @@
+#!/bin/bash
+# K3s agent join script for netboot nodes
+# Fetches token from server and starts k3s agent
+#
+# Runs at boot via k3s-join.service
+
+set -euo pipefail
+
+K3S_SERVER="192.168.100.1"
+K3S_URL="https://${K3S_SERVER}:6443"
+TOKEN_URL="http://${K3S_SERVER}:8800/k3s-token"
+MAX_RETRIES=30
+RETRY_DELAY=10
+
+# Colors for console output
+RED='\033[0;31m'
+GREEN='\033[0;32m'
+YELLOW='\033[1;33m'
+NC='\033[0m'
+
+log()   { echo -e "${GREEN}[k3s-join]${NC} $*"; logger -t k3s-join "$*"; }
+warn()  { echo -e "${YELLOW}[k3s-join]${NC} $*"; logger -t k3s-join -p warning "$*"; }
+error() { echo -e "${RED}[k3s-join]${NC} $*"; logger -t k3s-join -p err "$*"; }
+
+# Wait for network to be ready
+wait_for_network() {
+    local count=0
+    while ! ping -c1 -W1 "$K3S_SERVER" &>/dev/null; do
+        count=$((count + 1))
+        if [ $count -ge $MAX_RETRIES ]; then
+            error "Network not available after $MAX_RETRIES attempts"
+            return 1
+        fi
+        warn "Waiting for network... ($count/$MAX_RETRIES)"
+        sleep $RETRY_DELAY
+    done
+    log "Network is up"
+}
+
+# Fetch join token from server
+fetch_token() {
+    local count=0
+    local token=""
+    
+    while [ -z "$token" ]; do
+        token=$(curl -sf "$TOKEN_URL" 2>/dev/null || true)
+        if [ -z "$token" ]; then
+            count=$((count + 1))
+            if [ $count -ge $MAX_RETRIES ]; then
+                error "Failed to fetch token after $MAX_RETRIES attempts"
+                return 1
+            fi
+            warn "Waiting for token... ($count/$MAX_RETRIES)"
+            sleep $RETRY_DELAY
+        fi
+    done
+    
+    echo "$token"
+}
+
+# Main
+log "Starting K3s agent join process"
+
+wait_for_network
+
+log "Fetching join token from $TOKEN_URL"
+K3S_TOKEN=$(fetch_token)
+if [ -z "$K3S_TOKEN" ]; then
+    error "Failed to get token, exiting"
+    exit 1
+fi
+log "Token acquired"
+
+log "Starting K3s agent (server: $K3S_URL)"
+exec /usr/local/bin/k3s agent \
+    --server="$K3S_URL" \
+    --token="$K3S_TOKEN" \
+    --node-name="$(hostname)"