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/model/types.go

@@ -41,6 +41,7 @@ type DaemonConfig struct {
 	SSHKeyPath          string
 	NamegenPath         string
 	CustomizeScript     string
+	VSockPingHelperPath string
 	AutoStopStaleAfter  time.Duration
 	StatsPollInterval   time.Duration
 	MetricsPollInterval time.Duration
@@ -87,6 +88,8 @@ type VMRuntime struct {
 	GuestIP       string  `json:"guest_ip"`
 	TapDevice     string  `json:"tap_device,omitempty"`
 	APISockPath   string  `json:"api_sock_path,omitempty"`
+	VSockPath     string  `json:"vsock_path,omitempty"`
+	VSockCID      uint32  `json:"vsock_cid,omitempty"`
 	LogPath       string  `json:"log_path,omitempty"`
 	MetricsPath   string  `json:"metrics_path,omitempty"`
 	DNSName       string  `json:"dns_name,omitempty"`