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/firecracker/client_test.go

@@ -2,9 +2,13 @@ package firecracker
 
 import (
 	"bytes"
+	"context"
 	"log/slog"
+	"net"
+	"path/filepath"
 	"strings"
 	"testing"
+	"time"
 )
 
 func TestBuildConfig(t *testing.T) {
@@ -21,6 +25,8 @@ func TestBuildConfig(t *testing.T) {
 			{ID: "work", Path: "/var/lib/banger/root.ext4"},
 		},
 		TapDevice: "tap-fc-1",
+		VSockPath: "/tmp/fc.vsock",
+		VSockCID:  10042,
 		VCPUCount: 4,
 		MemoryMiB: 2048,
 	})
@@ -46,6 +52,12 @@ func TestBuildConfig(t *testing.T) {
 	if len(cfg.NetworkInterfaces) != 1 {
 		t.Fatalf("interface count = %d, want 1", len(cfg.NetworkInterfaces))
 	}
+	if len(cfg.VsockDevices) != 1 {
+		t.Fatalf("vsock count = %d, want 1", len(cfg.VsockDevices))
+	}
+	if cfg.VsockDevices[0].Path != "/tmp/fc.vsock" || cfg.VsockDevices[0].CID != 10042 {
+		t.Fatalf("unexpected vsock config: %+v", cfg.VsockDevices[0])
+	}
 	if got := cfg.NetworkInterfaces[0].StaticConfiguration.HostDevName; got != "tap-fc-1" {
 		t.Fatalf("host dev name = %q", got)
 	}
@@ -115,3 +127,81 @@ func TestSDKLoggerBridgeSuppressesDebugAtInfoLevel(t *testing.T) {
 		t.Fatalf("expected info-level logger to suppress sdk debug chatter, got %q", buf.String())
 	}
 }
+
+func TestPingVSock(t *testing.T) {
+	dir := t.TempDir()
+	socketPath := filepath.Join(dir, "fc.vsock")
+	listener, err := net.Listen("unix", socketPath)
+	if err != nil {
+		t.Fatalf("Listen: %v", err)
+	}
+	defer listener.Close()
+
+	done := make(chan error, 1)
+	go func() {
+		conn, err := listener.Accept()
+		if err != nil {
+			done <- err
+			return
+		}
+		defer conn.Close()
+		buf := make([]byte, 0, 64)
+		tmp := make([]byte, 64)
+		for {
+			n, err := conn.Read(tmp)
+			if err != nil {
+				done <- err
+				return
+			}
+			buf = append(buf, tmp[:n]...)
+			if strings.Contains(string(buf), "\n") {
+				break
+			}
+		}
+		if got := string(buf); got != "CONNECT 42070\n" {
+			done <- errUnexpectedString(got)
+			return
+		}
+		if _, err := conn.Write([]byte("OK 55\n")); err != nil {
+			done <- err
+			return
+		}
+		buf = buf[:0]
+		for {
+			n, err := conn.Read(tmp)
+			if err != nil {
+				done <- err
+				return
+			}
+			buf = append(buf, tmp[:n]...)
+			if strings.Contains(string(buf), "\n") {
+				break
+			}
+		}
+		if got := string(buf); got != "PING\n" {
+			done <- errUnexpectedString(got)
+			return
+		}
+		_, err = conn.Write([]byte("PONG\n"))
+		done <- err
+	}()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
+	defer cancel()
+	if err := PingVSock(ctx, nil, socketPath); err != nil {
+		t.Fatalf("PingVSock: %v", err)
+	}
+	if err := <-done; err != nil {
+		t.Fatalf("server: %v", err)
+	}
+}
+
+type unexpectedStringError string
+
+func (e unexpectedStringError) Error() string {
+	return "unexpected string: " + string(e)
+}
+
+func errUnexpectedString(value string) error {
+	return unexpectedStringError(value)
+}