[#699] split k8s platform body of work out of PFVCluster

Framework enforcement layer adopted; shellcheck clean; real kubeconfigs
gitignored (example only in repo). Provenance: PFVCluster/k8s.
https://projects.knownelement.com/issues/699#note-4536

probe-nodes.sh

@@ -0,0 +1,16 @@
+#!/usr/bin/bash
+# k8s/probe-nodes.sh — verify SSH + Tailscale reachability on all cnodes
+set -uo pipefail
+source "$(cd "$(dirname "$0")" && pwd)/env.sh"
+
+for ip in "${ALL_CNODES[@]}"; do
+  echo "=== $ip ==="
+  ssh "${SSH_OPTS[@]}" "${SSH_USER}@${ip}" \
+    'echo "host=$(hostname)"; \
+     echo "tailscale_ip=$(tailscale ip -4)"; \
+     echo "k3s=$(systemctl is-active k3s 2>/dev/null)"; \
+     echo "kernel=$(uname -r)"; \
+     free -h | head -2; \
+     df -h / | tail -1' 2>&1 || echo "FAILED to reach $ip"
+  echo
+done