feat(vm): derive guest rootfs from sandbox images

[drew]

Summary

Update the VM compute driver so sandbox images drive guest rootfs creation, including per-image caching and Dockerless OCI fetch/unpack for registry and community image refs.
Keep Docker only for local Dockerfile builds on the CLI host, where the CLI exports a rootfs artifact for the VM backend, and update the VM gateway scripts/docs to match the new flow.

Related Issue

N/A

Changes

• remove the bundled VM rootfs fallback and require a sandbox image or configured default image for VM sandboxes
• add OCI-based image fetch, layer unpack, whiteout handling, and digest-keyed rootfs caching in openshell-driver-vm
• keep Dockerfile sources on the host Docker socket, export them as local rootfs tar artifacts for VM sandboxes, and retain gateway-container imports for local Kubernetes gateways
• bundle the guest supervisor separately, update gateway:vm setup/start behavior, and refresh VM architecture and user docs
• fix VM rootfs archiving to preserve broken symlinks found in real container filesystems

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Additional testing run:

• cargo check -p openshell-driver-vm -p openshell-cli -p openshell-bootstrap
• cargo test -p openshell-bootstrap encode_and_decode_rootfs_tar_image_ref_round_trip
• cargo test -p openshell-driver-vm apply_layer_dir_to_rootfs_honors_whiteouts
• cargo test -p openshell-driver-vm layer_compression_from_media_type_supports_common_formats
• cargo test -p openshell-driver-vm compute_file_sha256_returns_prefixed_digest
• cargo test -p openshell-driver-vm create_rootfs_archive_preserves_broken_symlinks
• cargo test -p openshell-driver-vm prepare_sandbox_rootfs_rewrites_guest_layout
• attempted mise run pre-commit, but test:rust failed in sccache with Too many open files while compiling ring

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

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