[Kubernetes] Add explicit external OpenShell target and readiness

Parent: #9816

## Outcome

A non-root NemoClaw Blueprint Runner can target one externally managed OpenShell gateway and workspace without relying on ambient CLI state or local gateway lifecycle management.

## Scope

- Define one typed external target with an HTTPS endpoint, OpenShell workspace, Secret-mounted trust and machine-authentication file references, compatible release, and external lifecycle ownership.
- Route plan, readiness, inventory, version checks, and status through the exact target.
- Persist only sanitized target identity: endpoint, workspace, authentication kind, CA fingerprint, and compatibility result.
- Reject mixed local/external lifecycle configuration before any remote or local effect.
- Do not install, start, stop, restart, upgrade, or remove the gateway.

## Acceptance

- [ ] Valid explicit inputs let a non-root Pod or Job verify the exact gateway and workspace.
- [ ] Missing or invalid endpoint, CA, authentication, workspace, or release fails before mutation with redacted diagnostics.
- [ ] No credential, key, token, or private path value appears in arguments, logs, plans, receipts, or artifacts.
- [ ] Tests prove zero Docker, Podman, and local gateway lifecycle calls.
- [ ] The existing same-host externally supervised gateway behavior remains unchanged.

## Dependencies

Coordinate with #9802, #9803, #9807, and #9812. Prefer SDK or direct gRPC operations; any CLI-backed implementation must provide equivalent explicit targeting and noninteractive authentication.

## Out of scope

Sandbox creation, provider/policy/inference mutation, agent execution, and live Kubernetes qualification.