policy: Add policy to prevent host network usage

The `policy` Kustomize project defines various cluster-wide security
policies.  Initially, this includes a Validating Admission Policy that
prevents pods from using the host's network namespace.

policy/README.md

@@ -0,0 +1,30 @@
+# Cluster Policies
+
+## Validating Admission Policy
+
+To enable (prior to Kubernetes v1.30):
+
+1. Add the following to `apiServer.extraArgs` in the `ClusterConfiguration` key
+   of the `kubeadm-config` ConfigMap:
+
+   ```yaml
+   feature-gates: ValidatingAdmissionPolicy=true
+   runtime-config: admissionregistration.k8s.io/v1beta1=true
+   ```
+2. Redeploy the API servers using `kubeadm`:
+
+   ```sh
+   doas kubeadm upgrade apply v1.29.15 --yes
+   ```
+
+
+### disallow-hostnetwork
+
+This policy prevents pods from running in the host's network namespace. This is
+especially important because most nodes are connected to the storage network
+VLAN, so allowing pods to use the host network namespace would give them access
+to the iSCSI LUNs and NFS shares on the NAS.
+
+If a trusted pod needs to run in the host's network namespace, its Kubernetes
+namespace can be listed in the exclusion list of the
+`disallow-hostnetwork-binding` policy binding resource.