feat(k8s): Support ImageVolumeSource for supervisor binary sideload

[mrunalp]

feat(k8s): Support ImageVolumeSource for supervisor binary sideload

Problem Statement

The Kubernetes driver delivers the supervisor binary into sandbox pods using an
initContainer + emptyDir pattern: an init container runs copy-self to write
the binary from the supervisor image into an emptyDir volume, then the agent
container mounts it read-only. This approach has three drawbacks:

1. Startup latency — the init container must complete before the agent
   container can start, adding a sequential copy step to every sandbox launch.
2. Complexity — three moving parts (emptyDir volume, init container with
   copy-self subcommand, volume mount) make the pod spec harder to debug.
3. Redundancy — Kubernetes ImageVolumeSource (GA in v1.36) can mount an OCI
   image directly as a read-only volume, making the copy step unnecessary.

Proposed Design

Add a SupervisorSideloadMethod enum with two variants and thread it through
the driver config, CLI args, Helm chart, and pod template construction:

• image-volume (default) — emits a Kubernetes image volume source that
  mounts the supervisor OCI image directly. No init container or emptyDir
  needed. The binary is available at the same path
  (/opt/openshell/bin/openshell-sandbox) because the supervisor image has
  /openshell-sandbox at its root, and the image volume is mounted at
  /opt/openshell/bin.

• init-container (fallback) — preserves the existing emptyDir + init
  container pattern for clusters running Kubernetes < v1.33.

Components changed

File	Change
crates/openshell-driver-kubernetes/src/config.rs	SupervisorSideloadMethod enum with FromStr/Display/Default
crates/openshell-driver-kubernetes/src/driver.rs	supervisor_image_volume() helper, branch in apply_supervisor_sideload()
crates/openshell-driver-kubernetes/src/main.rs	--supervisor-sideload-method CLI arg / OPENSHELL_SUPERVISOR_SIDELOAD_METHOD env var
crates/openshell-server/src/lib.rs	Wire env var into server config
deploy/helm/openshell/values.yaml	supervisor.sideloadMethod value
deploy/helm/openshell/templates/statefulset.yaml	Pass env var to driver

Volume JSON emitted (image-volume)

{
  "name": "openshell-supervisor-bin",
  "image": {
    "reference": "openshell/supervisor:latest",
    "pullPolicy": "IfNotPresent"
  }
}

What stays the same

Agent container modifications (command override to
/opt/openshell/bin/openshell-sandbox, runAsUser: 0, read-only volume mount)
are identical for both methods. No changes to k8s-openapi dependency — volumes
are built as raw serde_json::json!().

Alternatives Considered

1. Switch entirely to ImageVolumeSource, drop initContainer — simpler but
   would break clusters running Kubernetes < v1.33. The fallback path costs
   almost nothing to maintain since the existing helper functions are unchanged.

2. Auto-detect via API server version — query the cluster version at runtime
   and choose automatically. Rejected because it adds complexity, and operators
   should make an explicit choice about which K8s features their cluster
   supports. A misconfigured image-volume on an older cluster produces a clear
   Kubernetes API error.

3. Use subPath to mount only the binary — subPath for image volumes was
   added in the v1.33 beta. Not needed because mounting the entire supervisor
   image at /opt/openshell/bin already places the binary at the correct path,
   and the supervisor image is FROM scratch with only the binary.

Agent Investigation

• Explored crates/openshell-driver-kubernetes/src/driver.rs — supervisor
  sideload is implemented in apply_supervisor_sideload() (line 741) with
  helpers supervisor_volume(), supervisor_init_container(), and
  supervisor_volume_mount(). All volumes are built as raw JSON via
  serde_json::json!(), so no k8s-openapi upgrade is needed.
• Confirmed the supervisor image places the binary at /openshell-sandbox
  (filesystem root). Mounting the image volume at /opt/openshell/bin makes
  the binary available at /opt/openshell/bin/openshell-sandbox — same path
  as the initContainer approach.
• Verified k8s-openapi is at v0.21.1 with v1_26 feature and does not
  include ImageVolumeSource types, confirming the raw JSON approach is
  necessary.
• Tested end-to-end on a local Kubernetes v1.37 / CRI-O 1.36 cluster. Sandbox
  pod reached 1/1 Running with the image volume source and no supervisor init
  container. See architecture/plans/image-volume-sideload-testing.md for full
  test procedure and results.

Checklist

• I've reviewed existing issues and the architecture docs
• This is a design proposal, not a "please build this" request