feat(k8s): deploy 3-node k3s HA control plane over Tailscale

Bootstrap a regular (non-ITAR) k3s cluster on cnode1/2/3 with embedded
etcd. All cluster communication — node registration, API server, etcd
peering, flannel VXLAN — runs exclusively over Tailscale IPs. Zero LAN
addresses appear in node status or TLS certificates.

Scripts (k8s/):
- env.sh: shared config (Tailscale IPs, SSH opts, k3s version)
- wipe.sh: remove existing k3s from all cnodes
- install-cp.sh: full bootstrap (cnode1 --cluster-init, then cnode2/3 join)
- join-servers.sh: re-join cnode2/3 only (fixes broken join state)
- post-setup.sh: apply NoSchedule taints, fetch kubeconfig, verify
- verify.sh: 13-point health check (nodes, Tailscale IPs, taints, etcd,
  CoreDNS, API server, workload isolation)
- probe-nodes.sh: SSH + Tailscale reachability check

All 3 cnodes are tainted control-plane:NoSchedule so no user workloads
can schedule on the control plane. 13/13 health checks pass.

Docs updated: k8s README TL;DR reflects k3s (not Talos) as the deployed
choice, with Talos preserved for the future ITAR cluster.

💘 Generated with Crush

Assisted-by: Crush:glm-5.2

k8s/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