feat(driver-vm): snapshot/restore (suspend/resume) for idle sandboxes

agent sandboxes are idle most of the time, waiting on tool calls, model responses, or humans. right now the ComputeDriver lifecycle is create/stop/delete, so an idle sandbox either holds its resources or gets torn down and cold-restarted (libkrun first boot is ~10-30s). what i'd want is suspend/resume: checkpoint a sandbox's memory + fs, free the resources, and restore it in well under a second on the next request, same idle economics Lambda gets from Firecracker snapshots.

a few questions:

• is VM snapshot/restore on the roadmap at all? i see the VmBackend { Libkrun, Qemu } split and the QEMU backend from #992 (plus the closed CH attempt in #851), so the backend abstraction already exists.
• would adding Snapshot/Restore RPCs to ComputeDriver (alongside create/stop/delete) be welcome?
• preferred path: snapshot on the QEMU backend (savevm/migrate-to-file), or a dedicated Firecracker / Cloud Hypervisor backend? both have first-class snapshot/restore.
• or is this intentionally deferred behind the multi-tenant milestone?

context: evaluating OpenShell as the sandbox runtime for an agent-compute substrate where suspend/resume + KV cache aware routing/offloading are the core economics. happy to help scope or send a patch if there's appetite.

Start with the ComputeDriver lifecycle and the VmBackend split described in the issue, then review the QEMU work in #992 and the closed CH attempt in #851. Determine whether Snapshot/Restore belongs in the driver abstraction and which backend path is supported; done means a decided scope or roadmap direction rather than an implementation patch.