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/vsockping/vsockping.go

@@ -0,0 +1,105 @@
+package vsockping
+
+import (
+	"bufio"
+	"context"
+	"fmt"
+	"io"
+	"log/slog"
+	"net"
+	"strings"
+	"time"
+
+	sdkvsock "github.com/firecracker-microvm/firecracker-go-sdk/vsock"
+)
+
+const (
+	Port             uint32 = 42070
+	RequestLine             = "PING\n"
+	ResponseLine            = "PONG\n"
+	GuestBinaryName         = "banger-vsock-pingd"
+	GuestInstallPath        = "/usr/local/bin/" + GuestBinaryName
+	ServiceName             = "banger-vsock-pingd.service"
+	serviceUnit             = `[Unit]
+Description=Banger vsock ping responder
+After=network.target
+
+[Service]
+Type=simple
+ExecStart=/usr/local/bin/banger-vsock-pingd
+Restart=on-failure
+RestartSec=1
+
+[Install]
+WantedBy=multi-user.target
+`
+	modulesLoadConfig = "vsock\nvmw_vsock_virtio_transport\n"
+)
+
+func Ping(ctx context.Context, logger *slog.Logger, socketPath string) error {
+	conn, err := sdkvsock.DialContext(
+		ctx,
+		socketPath,
+		Port,
+		sdkvsock.WithRetryTimeout(3*time.Second),
+		sdkvsock.WithRetryInterval(100*time.Millisecond),
+	)
+	if err != nil {
+		return err
+	}
+	defer conn.Close()
+
+	if deadline, ok := ctx.Deadline(); ok {
+		_ = conn.SetDeadline(deadline)
+	} else {
+		_ = conn.SetDeadline(time.Now().Add(3 * time.Second))
+	}
+
+	if _, err := io.WriteString(conn, RequestLine); err != nil {
+		return err
+	}
+	line, err := bufio.NewReader(conn).ReadString('\n')
+	if err != nil {
+		return err
+	}
+	if strings.TrimSpace(line) != strings.TrimSpace(ResponseLine) {
+		return fmt.Errorf("unexpected vsock ping response %q", strings.TrimSpace(line))
+	}
+	if logger != nil {
+		logger.Debug("vsock ping ok", "vsock_path", socketPath, "vsock_port", Port)
+	}
+	return nil
+}
+
+func ServiceUnit() string {
+	return serviceUnit
+}
+
+func ModulesLoadConfig() string {
+	return modulesLoadConfig
+}
+
+func ReminderMessage(name string) string {
+	return fmt.Sprintf("session ended; %s is still running (stop it with 'banger vm stop %s')", name, name)
+}
+
+func WarningMessage(name string, err error) string {
+	if err == nil {
+		return ""
+	}
+	return fmt.Sprintf("warning: failed to check whether %s is still running: %v", name, err)
+}
+
+func ServeConn(conn net.Conn) error {
+	defer conn.Close()
+	_ = conn.SetDeadline(time.Now().Add(5 * time.Second))
+	line, err := bufio.NewReader(conn).ReadString('\n')
+	if err != nil {
+		return err
+	}
+	if strings.TrimSpace(line) != strings.TrimSpace(RequestLine) {
+		return fmt.Errorf("unexpected request %q", strings.TrimSpace(line))
+	}
+	_, err = io.WriteString(conn, ResponseLine)
+	return err
+}