Add unikernel runtime: run services as Nanos VMs under QEMU/KVM

Implements the hypervisor design's Phase 1: a second runtime.Runtime
backend (QEMU) that runs each service component as a Nanos unikernel VM
instead of a podman container, selected per-component via a new
runtime = "unikernel" service-def field.

- internal/runtime/qemu.go: QEMURuntime. Pull extracts the ELF from the
  OCI image; Run does `ops build` + boots qemu-system-x86_64 with KVM,
  user-mode net port-forwards, QMP control socket and serial console log;
  Stop/Remove/Inspect/List/Logs map onto VM lifecycle + state dir.
- proto/registry/servicedef: add runtime, memory_mb, vcpus fields
  (registry migration 5).
- agent: holds both runtimes; runtimeFor() selects per component;
  listAllContainers() merges containers + VMs so drift/status see both.
  Unikernel runtime auto-enables on nodes with /dev/kvm + ops.

Validated end-to-end on straylight: a test service deploys via
`mcp deploy --direct`, boots as a Nanos unikernel, serves HTTP through
the agent port-forward, and reports running via `mcp status`/`mcp logs`.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/master/status.go

@@ -117,7 +117,7 @@ func (m *Master) ListNodes(_ context.Context, _ *mcpv1.ListNodesRequest) (*mcpv1
 			Address:    n.Address,
 			Arch:       n.Arch,
 			Status:     n.Status,
-			Containers: int32(n.Containers), //nolint:gosec // small number
+			Containers: int32(n.Containers),   //nolint:gosec // small number
 			Services:   int32(counts[n.Name]), //nolint:gosec // small number
 		}
 		if n.LastHeartbeat != nil {