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.

cmd/banger-vsock-pingd/main.go

@@ -0,0 +1,49 @@
+package main
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"net"
+	"os"
+	"os/signal"
+	"syscall"
+	"time"
+
+	sdkvsock "github.com/firecracker-microvm/firecracker-go-sdk/vsock"
+	"github.com/sirupsen/logrus"
+
+	"banger/internal/vsockping"
+)
+
+func main() {
+	ctx, cancel := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
+	defer cancel()
+
+	logger := logrus.New()
+	logger.SetOutput(io.Discard)
+	listener, err := sdkvsock.Listener(ctx, logrus.NewEntry(logger), vsockping.Port)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "banger-vsock-pingd: %v\n", err)
+		os.Exit(1)
+	}
+	defer listener.Close()
+
+	for {
+		conn, err := listener.Accept()
+		if err != nil {
+			if ctx.Err() != nil || errors.Is(err, net.ErrClosed) {
+				return
+			}
+			fmt.Fprintf(os.Stderr, "banger-vsock-pingd: accept: %v\n", err)
+			time.Sleep(200 * time.Millisecond)
+			continue
+		}
+		go func(conn net.Conn) {
+			if err := vsockping.ServeConn(conn); err != nil {
+				fmt.Fprintf(os.Stderr, "banger-vsock-pingd: %v\n", err)
+			}
+		}(conn)
+	}
+}