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>

.dsh/skills/dsh-agent-sandbox-platform/SKILL.md

@@ -129,6 +129,10 @@ Read `docs/examples.md` and the testing table in `AGENTS.md` before editing.
   `make check` and the e2e. Never mix a bump with an unrelated feature.
 - **MCP integration:** pin the build to the cluster's tag, least-privilege SA,
   ClusterIP + auth proxy, raised DSH timeout, no secrets in the repo.
+- **Optional hardening:** gVisor, `nodeSelector`, NetworkPolicy and extra
+  provider keys stay opt-in; the sandbox policy lives under
+  `manifests/optional/`. All four are documented in
+  `examples/dsh-agents-as-a-service/docs/optional-hardening.md`.
 - **Docs:** update the affected doc in the same change.
 
 Details and review checklist:

.dsh/skills/dsh-agent-sandbox-platform/references/extending-the-repo.md

@@ -25,6 +25,7 @@ implementation is a self-contained example under `examples/<name>/`, and
 | --- | --- | --- |
 | Documentation only | `*.md`, `docs/` | `make check`; relative links resolve |
 | Example manifest | `examples/<name>/manifests/NN-*.yaml`, README step | `make check`; server-side dry-run in CI |
+| Optional hardening | `manifests/optional/`, exposure NPs, example `docs/optional-hardening.md` | `make check`; manual verification on a capable cluster |
 | Gateway behavior/route | `gateway/gateway.py`, `gateway/test_gateway.py`, gateway README, architecture | `make test`, `make lint`; update tests and API docs |
 | Exposure variant | `manifests/exposure/<variant>/`, exposure README, example README | `make check`; apply exactly one variant |
 | New example | full `examples/<name>/` layout, `examples/README.md` | `make check`; CI dry-run; optional e2e |

.dsh/skills/dsh-agent-sandbox-platform/references/platform-runbook.md

@@ -141,8 +141,13 @@ host. A resource-only install has no gateway: that is a warning, not a failure.
 
 ## 9. Security boundaries
 
-- One `DEEPSEEK_API_KEY` is shared by all sandboxes; the example is not
+- Provider keys (`dsh-provider-keys` plus the optional
+  `dsh-provider-keys-extra`) are shared by all sandboxes; the example is not
   tenant-isolated.
+- gVisor, `nodeSelector`, the sandbox/exposure NetworkPolicies and extra
+  provider keys are opt-in, not active by default. A NetworkPolicy only helps
+  with a CNI that enforces it; see
+  [optional hardening](../../../../examples/dsh-agents-as-a-service/docs/optional-hardening.md).
 - Secret data, kubeconfigs, and pod logs are sensitive; RBAC may allow Secret
   access, policy does not. Keep gateway RBAC namespace-scoped; it needs pod logs
   to find the launch token.