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/tui_test.go

@@ -2,12 +2,14 @@ package cli
 
 import (
 	"context"
+	"errors"
 	"os"
 	"path/filepath"
 	"strings"
 	"testing"
 	"time"
 
+	"banger/internal/api"
 	"banger/internal/model"
 	"banger/internal/paths"
 
@@ -236,6 +238,41 @@ func TestTUIStatusIncludesStageDurationsAfterInitialLoad(t *testing.T) {
 	}
 }
 
+func TestSSHDoneMsgShowsReminderWhenPingAlive(t *testing.T) {
+	origPing := vmPingFunc
+	t.Cleanup(func() {
+		vmPingFunc = origPing
+	})
+	vmPingFunc = func(ctx context.Context, socketPath, idOrName string) (api.VMPingResult, error) {
+		return api.VMPingResult{Name: "devbox", Alive: true}, nil
+	}
+
+	msg := sshDoneMsg(paths.Layout{SocketPath: "/tmp/bangerd.sock"}, actionRequest{id: "devbox", name: "devbox"}, "devbox", nil)
+	result, ok := msg.(actionResultMsg)
+	if !ok {
+		t.Fatalf("msg = %T, want actionResultMsg", msg)
+	}
+	if !strings.Contains(result.status, "devbox is still running") {
+		t.Fatalf("status = %q, want reminder", result.status)
+	}
+}
+
+func TestSSHDoneMsgShowsWarningWhenPingFails(t *testing.T) {
+	origPing := vmPingFunc
+	t.Cleanup(func() {
+		vmPingFunc = origPing
+	})
+	vmPingFunc = func(ctx context.Context, socketPath, idOrName string) (api.VMPingResult, error) {
+		return api.VMPingResult{}, errors.New("dial failed")
+	}
+
+	msg := sshDoneMsg(paths.Layout{SocketPath: "/tmp/bangerd.sock"}, actionRequest{id: "devbox", name: "devbox"}, "devbox", nil)
+	result := msg.(actionResultMsg)
+	if !strings.Contains(result.status, "failed to check whether devbox is still running") {
+		t.Fatalf("status = %q, want warning", result.status)
+	}
+}
+
 func TestAggregateRunningVMResources(t *testing.T) {
 	t.Parallel()