feat(example): add optional gVisor, nodeSelector, NetworkPolicy and provider keys (#21)

Add opt-in hardening to the agents-as-a-service example:
- commented runtimeClassName/nodeSelector fields for sandboxes and Deployments
- optional sandbox NetworkPolicy plus exposure-variant policies
- optional dsh-provider-keys-extra Secret for extra provider keys
- docs/optional-hardening.md with verification, caveats and rollback

The base walkthrough stays portable; no gateway code, routes or auth changed.

Co-authored-by: SREDevOpsAgents <agents@sredevops.org>

examples/dsh-agents-as-a-service/manifests/exposure/ingress/60-ingress.yaml

@@ -23,6 +23,10 @@
 # middlewares (e.g. compress, buffering) to this router — they break streaming
 # and upgrades. `path: /` Prefix covers the landing page and every per-user
 # proxy path, so no per-sandbox Ingress is needed.
+#
+# 61-networkpolicy.yaml is part of this variant: it restricts the gateway's
+# ingress to Traefik pods. Adjust its selector if your ingress controller uses
+# a different label or hostNetwork.
 apiVersion: networking.k8s.io/v1
 kind: Ingress
 metadata:

examples/dsh-agents-as-a-service/manifests/exposure/ingress/61-networkpolicy.yaml

@@ -0,0 +1,31 @@
+# OPTIONAL with the Ingress exposure variant: restrict the gateway's ingress
+# to the ingress controller.
+#
+# This manifest assumes Traefik, matching this variant's
+# `ingressClassName: traefik`. Traefik's Helm chart labels its pods
+# app.kubernetes.io/name=traefik; adjust the selector for another controller or
+# for a hostNetwork install (where the source is the node IP, not a pod IP).
+#
+# NetworkPolicy enforcement of kubelet probe traffic is CNI-dependent. If the
+# gateway starts failing its /healthz probes, allow the node CIDR on port 8080
+# or remove this policy. See ../../../docs/optional-hardening.md.
+apiVersion: networking.k8s.io/v1
+kind: NetworkPolicy
+metadata:
+  name: dsh-gateway-ingress
+  namespace: dsh-demo
+spec:
+  podSelector:
+    matchLabels:
+      app: dsh-gateway
+  policyTypes:
+    - Ingress
+  ingress:
+    - from:
+        - namespaceSelector: {}
+          podSelector:
+            matchLabels:
+              app.kubernetes.io/name: traefik
+      ports:
+        - protocol: TCP
+          port: 8080