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/cli/cli_test.go

@@ -4,7 +4,9 @@ import (
 	"bytes"
 	"context"
 	"errors"
+	"io"
 	"os"
+	"os/exec"
 	"path/filepath"
 	"reflect"
 	"strings"
@@ -209,6 +211,56 @@ func TestVMSetParamsFromFlagsRejectsNonPositiveCPUAndMemory(t *testing.T) {
 	}
 }
 
+func TestRunSSHSessionPrintsReminderWhenPingAlive(t *testing.T) {
+	origSSHExec := sshExecFunc
+	origPing := vmPingFunc
+	t.Cleanup(func() {
+		sshExecFunc = origSSHExec
+		vmPingFunc = origPing
+	})
+
+	sshExecFunc = func(ctx context.Context, stdin io.Reader, stdout, stderr io.Writer, args []string) error {
+		return nil
+	}
+	vmPingFunc = func(ctx context.Context, socketPath, idOrName string) (api.VMPingResult, error) {
+		return api.VMPingResult{Name: "devbox", Alive: true}, nil
+	}
+
+	var stderr bytes.Buffer
+	if err := runSSHSession(context.Background(), "/tmp/bangerd.sock", "devbox", strings.NewReader(""), &bytes.Buffer{}, &stderr, []string{"root@127.0.0.1"}); err != nil {
+		t.Fatalf("runSSHSession: %v", err)
+	}
+	if !strings.Contains(stderr.String(), "devbox is still running") {
+		t.Fatalf("stderr = %q, want reminder", stderr.String())
+	}
+}
+
+func TestRunSSHSessionPreservesSSHExitStatusOnPingWarning(t *testing.T) {
+	origSSHExec := sshExecFunc
+	origPing := vmPingFunc
+	t.Cleanup(func() {
+		sshExecFunc = origSSHExec
+		vmPingFunc = origPing
+	})
+
+	sshExecFunc = func(ctx context.Context, stdin io.Reader, stdout, stderr io.Writer, args []string) error {
+		return &exec.ExitError{}
+	}
+	vmPingFunc = func(ctx context.Context, socketPath, idOrName string) (api.VMPingResult, error) {
+		return api.VMPingResult{}, errors.New("dial failed")
+	}
+
+	var stderr bytes.Buffer
+	err := runSSHSession(context.Background(), "/tmp/bangerd.sock", "devbox", strings.NewReader(""), &bytes.Buffer{}, &stderr, []string{"root@127.0.0.1"})
+	var exitErr *exec.ExitError
+	if !errors.As(err, &exitErr) {
+		t.Fatalf("runSSHSession error = %v, want exit error", err)
+	}
+	if !strings.Contains(stderr.String(), "failed to check whether devbox is still running") {
+		t.Fatalf("stderr = %q, want warning", stderr.String())
+	}
+}
+
 func TestResolveVMTargetsDeduplicatesAndReportsErrors(t *testing.T) {
 	vms := []model.VMRecord{
 		testCLIResolvedVM("alpha-id", "alpha"),