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/banger.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"io"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -23,6 +24,7 @@ import (
 	"banger/internal/rpc"
 	"banger/internal/system"
 	"banger/internal/vmdns"
+	"banger/internal/vsockping"
 
 	"github.com/spf13/cobra"
 )
@@ -32,7 +34,17 @@ var (
 	daemonExePath   = func(pid int) string {
 		return filepath.Join("/proc", fmt.Sprintf("%d", pid), "exe")
 	}
-	doctorFunc = daemon.Doctor
+	doctorFunc  = daemon.Doctor
+	sshExecFunc = func(ctx context.Context, stdin io.Reader, stdout, stderr io.Writer, args []string) error {
+		sshCmd := exec.CommandContext(ctx, "ssh", args...)
+		sshCmd.Stdout = stdout
+		sshCmd.Stderr = stderr
+		sshCmd.Stdin = stdin
+		return sshCmd.Run()
+	}
+	vmPingFunc = func(ctx context.Context, socketPath, idOrName string) (api.VMPingResult, error) {
+		return rpc.Call[api.VMPingResult](ctx, socketPath, "vm.ping", api.VMRefParams{IDOrName: idOrName})
+	}
 )
 
 func NewBangerCommand() *cobra.Command {
@@ -454,11 +466,7 @@ func newVMSSHCommand() *cobra.Command {
 			if err != nil {
 				return err
 			}
-			sshCmd := exec.CommandContext(cmd.Context(), "ssh", sshArgs...)
-			sshCmd.Stdout = cmd.OutOrStdout()
-			sshCmd.Stderr = cmd.ErrOrStderr()
-			sshCmd.Stdin = cmd.InOrStdin()
-			return sshCmd.Run()
+			return runSSHSession(cmd.Context(), layout.SocketPath, result.Name, cmd.InOrStdin(), cmd.OutOrStdout(), cmd.ErrOrStderr(), sshArgs)
 		},
 	}
 }
@@ -953,6 +961,36 @@ func validatePositiveSetting(label string, value int) error {
 	return nil
 }
 
+func runSSHSession(ctx context.Context, socketPath, vmRef string, stdin io.Reader, stdout, stderr io.Writer, sshArgs []string) error {
+	sshErr := sshExecFunc(ctx, stdin, stdout, stderr, sshArgs)
+	if !shouldCheckSSHReminder(sshErr) || ctx.Err() != nil {
+		return sshErr
+	}
+	pingCtx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
+	defer cancel()
+	ping, err := vmPingFunc(pingCtx, socketPath, vmRef)
+	if err != nil {
+		_, _ = fmt.Fprintln(stderr, vsockping.WarningMessage(vmRef, err))
+		return sshErr
+	}
+	if ping.Alive {
+		name := ping.Name
+		if strings.TrimSpace(name) == "" {
+			name = vmRef
+		}
+		_, _ = fmt.Fprintln(stderr, vsockping.ReminderMessage(name))
+	}
+	return sshErr
+}
+
+func shouldCheckSSHReminder(err error) bool {
+	if err == nil {
+		return true
+	}
+	var exitErr *exec.ExitError
+	return errors.As(err, &exitErr)
+}
+
 func sshCommandArgs(cfg model.DaemonConfig, guestIP string, extra []string) ([]string, error) {
 	if guestIP == "" {
 		return nil, errors.New("vm has no guest IP")