feat(kubernetes): add sidecar supervisor topology

[TaylorMutch]

Summary

Adds the Kubernetes sidecar supervisor topology after the combined-topology base from #2074.
The default combined topology remains unchanged, and sidecar is opt-in.

sidecar moves pod-level network enforcement and gateway forwarding into a dedicated supervisor sidecar. The agent container can run as the resolved sandbox UID/GID with runAsNonRoot, no privilege escalation, and all Linux capabilities dropped.

This draft intentionally includes the prerequisite UID/GID commits from #1973 for now. We expect to rebase those out after that work lands separately.

Runtime validation status:

• sidecar is experimental with Kata Containers.
• sidecar is known to fail with gVisor because it depends on pod-local network rule setup.

Sidecar mode preserves gateway session and SSH behavior, but intentionally runs the process supervisor in network-only mode. Filesystem policy, process privilege dropping, and process/binary identity checks are not applied in this mode.

Related Issue

References #1827, #981, #899, #1305.

Related PRs: #1973, #2074, #2016.

Changes

• Accept numeric sandbox process identities and propagate configurable sandbox UID/GID values.
• Resolve Kubernetes sandbox UID/GID from explicit config or OpenShift SCC annotations.
• Add the sidecar supervisor topology and the related processEnforcement and proxyUid configuration.
• Render sidecar-mode sandbox pods with a network init container, non-root supervisor sidecar, and unprivileged agent container.
• Add process-supervisor network-only behavior for sidecar mode while keeping SSH/session relay behavior intact.
• Add sidecar e2e Helm values and Skaffold profile support.
• Add a sidecar topology row to the branch Kubernetes E2E matrix.
• Document topology choice, architecture diagrams, permission model, RuntimeClass validation status, and network-only tradeoffs.

Testing

• git diff --check origin/main..feat/kubernetes-sidecar-topology-v2
• cargo check -p openshell-driver-kubernetes -p openshell-sandbox -p openshell-supervisor-process -p openshell-supervisor-network
• cargo test -p openshell-driver-kubernetes --lib
• cargo test -p openshell-supervisor-process --lib
• mise run helm:test
• markdownlint-cli2 docs/kubernetes/topology.mdx docs/kubernetes/setup.mdx
• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/branch-e2e.yml"); puts "ok"'
• mise run rust:lint
• Prior validation from feat(kubernetes): add sidecar and proxy-pod topology configurations #2016: HELM_K3S_LB_HOST_PORT=18080 mise run e2e:kubernetes:sidecar
• Prior validation from feat(kubernetes): add sidecar and proxy-pod topology configurations #2016: sidecar topology smoke-tested as experimental on Kata Containers; known to fail on gVisor.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)

[copy-pr-bot]

Auto-sync is disabled for draft pull requests in this repository. Workflows must be run manually.

Contributors can view more details about this message here.

[github-actions]

🌿 Preview your docs: https://nvidia-preview-pr-2076.docs.buildwithfern.com/openshell

[github-actions]

Label test:e2e applied, but pull-request/2076 is at {"messa while the PR head is 6276de3. A maintainer needs to comment /ok to test 6276de38ac92b65b0b24a68b8d09a5eed06f50d7 to refresh the mirror. Once the mirror catches up, re-run Branch E2E Checks from the Actions tab.