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/registry/db.go

@@ -156,4 +156,9 @@ var migrations = []string{
 		created_at       TEXT NOT NULL DEFAULT (datetime('now')),
 		updated_at       TEXT NOT NULL DEFAULT (datetime('now'))
 	);`,
+
+	// Migration 5: unikernel runtime support (per-component runtime + VM resources)
+	`ALTER TABLE components ADD COLUMN runtime TEXT NOT NULL DEFAULT 'container';
+	 ALTER TABLE components ADD COLUMN memory_mb INTEGER NOT NULL DEFAULT 0;
+	 ALTER TABLE components ADD COLUMN vcpus INTEGER NOT NULL DEFAULT 0;`,
 }