feat(sandbox): support rootfs tar as --from source for VM driver

[feloy]

Summary

• Add rootfs tar archive (.tar, .tar.gz, .tgz) as a new --from source for VM-driver sandboxes, enabling daemon-free sandbox creation from flat filesystem archives produced by docker export, podman export, or buildah mount + tar
• CLI detects the archive by file extension, validates the gateway is local and uses the VM compute driver, and passes the path via driver_config
• VM driver copies the tar into its staging area and feeds it into the existing rootfs extraction and ext4 disk creation pipeline, with mtime-based caching

Related Issue

Closes #2175

Changes

CLI (crates/openshell-cli/)

• New ResolvedSource::RootfsTar variant in resolve_from() with extension-based detection
• validate_rootfs_tar_source() checks local-gateway and VM-driver constraints via GetGatewayInfo RPC
• rootfs_tar_driver_config() and merge_driver_config() encode the tar path into driver_config
• Updated --from help text and error messages

VM driver (crates/openshell-driver-vm/)

• rootfs_tar_path field on VmSandboxDriverConfig
• ensure_prepared_rootfs_tar_disk() with double-checked cache locking and mtime-based identity
• Updated validate_sandbox(), create_sandbox(), prepare_runtime_images(), and reconciliation to accept rootfs tar as an alternative to image references
• Extracted bootstrap_image_ref_default() for reuse in rootfs tar and reconciliation paths

Docs (docs/sandboxes/manage-sandboxes.mdx)

• Updated --from documentation with rootfs tar example and VM-driver constraint

Tests

• 6 unit tests in run.rs: extension detection for .tar/.tar.gz/.tgz, missing archive rejection, filename_looks_like_rootfs_tar coverage
• E2E test (e2e/rust/tests/rootfs_tar.rs): builds a Docker image, exports a flat rootfs tar, creates a VM sandbox from it, and verifies a marker file

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated

Checklist

• Follows conventional commit format
• No proto schema changes required (driver_config carries the tar path)
• Scoped to VM driver only — Docker and Podman drivers unaffected
• Local-gateway constraint enforced (same as Dockerfile sources)
• User-facing docs updated
• DCO sign-off included

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