feat(podman): honor OCI image working directories

[matthewgrossman]

Summary

Extend the Docker/shared OCI working-directory support from #2530 to the Podman driver. Podman validates the original immutable image before its workspace volume masks the OCI workdir, then launches the sandbox at the validated path without expanding the image user's filesystem authority.

This PR is conventionally stacked on #2530 and should be reviewed after that Docker/shared base.

Related Issue

Part of #2526

Changes

• validate the OCI workdir in a minimal temporary probe container created from the same pinned image ID
• run the probe without the workspace volume, network, secrets, or sandbox token
• attest the validated image identity through driver-owned metadata before supervisor startup
• mount Podman persistence at the resolved OCI workdir and reject conflicting driver-config mounts
• add wait, log collection, cleanup, and failure diagnostics for the probe lifecycle
• cover Podman OCI identity, supplementary groups, nested volumes, and forged validation metadata
• update Podman driver, architecture, user, CI, and agent guidance

Testing

• mise run pre-commit passes
• cargo test -p openshell-driver-podman -p openshell-driver-docker -p openshell-supervisor-process -p openshell-sandbox passes
• cargo test -p openshell-server --features test-support passes
• E2E coverage added/updated for Podman OCI workdir and identity behavior

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture and user documentation updated

[github-actions]

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