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/api/types.go

@@ -63,6 +63,11 @@ type VMSSHResult struct {
 	GuestIP string `json:"guest_ip"`
 }
 
+type VMPingResult struct {
+	Name  string `json:"name"`
+	Alive bool   `json:"alive"`
+}
+
 type ImageBuildParams struct {
 	Name       string `json:"name,omitempty"`
 	BaseRootfs string `json:"base_rootfs,omitempty"`