Add vsock-backed SSH session reminders

Remind users when a VM is still running after 	hanger vm ssh exits instead of silently dropping them back to the host shell.\n\nAttach a Firecracker vsock device to each VM, persist the host vsock path/CID,\nadd a new guest-side banger-vsock-pingd responder to the runtime bundle and both\nimage-build paths, and expose a vm.ping RPC that the CLI and TUI call after SSH\nreturns. Doctor and start/build preflight now validate the helper plus\n/dev/vhost-vsock so the feature fails early and clearly.\n\nValidated with go mod tidy, bash -n customize.sh, git diff --check, make build,\nand GOCACHE=/tmp/banger-gocache go test ./... outside the sandbox because the\ndaemon tests need real Unix/UDP sockets. Rebuild the image/rootfs used for new\nVMs so the guest ping service is present.

internal/daemon/vm_test.go

@@ -261,6 +261,114 @@ func TestSetVMRejectsStoppedOnlyChangesForRunningVM(t *testing.T) {
 	}
 }
 
+func TestPingVMReturnsAliveForRunningGuest(t *testing.T) {
+	t.Parallel()
+
+	ctx := context.Background()
+	db := openDaemonStore(t)
+	apiSock := filepath.Join(t.TempDir(), "fc.sock")
+	fake := startFakeFirecrackerProcess(t, apiSock)
+	t.Cleanup(func() {
+		_ = fake.Process.Kill()
+		_ = fake.Wait()
+	})
+
+	vsockSock := filepath.Join(t.TempDir(), "fc.vsock")
+	listener, err := net.Listen("unix", vsockSock)
+	if err != nil {
+		t.Fatalf("listen vsock: %v", err)
+	}
+	t.Cleanup(func() {
+		_ = listener.Close()
+		_ = os.Remove(vsockSock)
+	})
+	serverDone := make(chan error, 1)
+	go func() {
+		conn, err := listener.Accept()
+		if err != nil {
+			serverDone <- err
+			return
+		}
+		defer conn.Close()
+		buf := make([]byte, 128)
+		n, err := conn.Read(buf)
+		if err != nil {
+			serverDone <- err
+			return
+		}
+		if got := string(buf[:n]); got != "CONNECT 42070\n" {
+			serverDone <- fmt.Errorf("unexpected connect message %q", got)
+			return
+		}
+		if _, err := conn.Write([]byte("OK 1\n")); err != nil {
+			serverDone <- err
+			return
+		}
+		n, err = conn.Read(buf)
+		if err != nil {
+			serverDone <- err
+			return
+		}
+		if got := string(buf[:n]); got != "PING\n" {
+			serverDone <- fmt.Errorf("unexpected ping payload %q", got)
+			return
+		}
+		_, err = conn.Write([]byte("PONG\n"))
+		serverDone <- err
+	}()
+
+	vm := testVM("alive", "image-alive", "172.16.0.41")
+	vm.State = model.VMStateRunning
+	vm.Runtime.State = model.VMStateRunning
+	vm.Runtime.PID = fake.Process.Pid
+	vm.Runtime.APISockPath = apiSock
+	vm.Runtime.VSockPath = vsockSock
+	vm.Runtime.VSockCID = 10041
+	if err := db.UpsertVM(ctx, vm); err != nil {
+		t.Fatalf("UpsertVM: %v", err)
+	}
+
+	runner := &scriptedRunner{
+		t: t,
+		steps: []runnerStep{
+			sudoStep("", nil, "chown", fmt.Sprintf("%d:%d", os.Getuid(), os.Getgid()), vsockSock),
+			sudoStep("", nil, "chmod", "600", vsockSock),
+		},
+	}
+	d := &Daemon{store: db, runner: runner}
+	result, err := d.PingVM(ctx, vm.Name)
+	if err != nil {
+		t.Fatalf("PingVM: %v", err)
+	}
+	if !result.Alive || result.Name != vm.Name {
+		t.Fatalf("PingVM result = %+v, want alive %s", result, vm.Name)
+	}
+	runner.assertExhausted()
+	if err := <-serverDone; err != nil {
+		t.Fatalf("server: %v", err)
+	}
+}
+
+func TestPingVMReturnsFalseForStoppedVM(t *testing.T) {
+	t.Parallel()
+
+	ctx := context.Background()
+	db := openDaemonStore(t)
+	vm := testVM("stopped-ping", "image-stopped", "172.16.0.42")
+	if err := db.UpsertVM(ctx, vm); err != nil {
+		t.Fatalf("UpsertVM: %v", err)
+	}
+
+	d := &Daemon{store: db}
+	result, err := d.PingVM(ctx, vm.Name)
+	if err != nil {
+		t.Fatalf("PingVM: %v", err)
+	}
+	if result.Alive {
+		t.Fatalf("PingVM result = %+v, want not alive", result)
+	}
+}
+
 func TestSetVMDiskResizeFailsPreflightWhenToolsMissing(t *testing.T) {
 	ctx := context.Background()
 	db := openDaemonStore(t)