feat(vm): boot sandboxes from ext4 root disks

[drew]

Summary

Boot VM sandboxes from per-sandbox ext4 root disk images instead of exposing a prepared host rootfs directory to the guest.

Related Issue

N/A

Changes

• Cache prepared VM root filesystems as rootfs.ext4 images keyed by source image identity and rootfs layout version.
• Copy the cached root disk per sandbox and launch libkrun/QEMU with that disk as the guest root filesystem.
• Move guest environment and TLS injection into the ext4 image, keep guest /sandbox ownership inside guest filesystem metadata, and update VM docs/e2e notes.

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)
• mise run e2e:vm passes

Checklist

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

[github-actions]

🌿 Preview your docs: https://nvidia-preview-pr-1263.docs.buildwithfern.com/openshell

[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.