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/paths/paths_test.go

@@ -56,19 +56,21 @@ func TestResolveRuntimeDirUsesSourceCheckoutRuntimeSubdir(t *testing.T) {
 func createRuntimeBundle(t *testing.T, runtimeDir string) {
 	t.Helper()
 	metadata := runtimebundle.BundleMetadata{
-		FirecrackerBin:  "bin/firecracker",
-		SSHKeyPath:      "keys/id_ed25519",
-		NamegenPath:     "bin/namegen",
-		CustomizeScript: "scripts/customize.sh",
-		DefaultPackages: "config/packages.apt",
-		DefaultRootfs:   "images/rootfs-docker.ext4",
-		DefaultKernel:   "kernels/vmlinux",
+		FirecrackerBin:      "bin/firecracker",
+		SSHKeyPath:          "keys/id_ed25519",
+		NamegenPath:         "bin/namegen",
+		CustomizeScript:     "scripts/customize.sh",
+		VSockPingHelperPath: "bin/banger-vsock-pingd",
+		DefaultPackages:     "config/packages.apt",
+		DefaultRootfs:       "images/rootfs-docker.ext4",
+		DefaultKernel:       "kernels/vmlinux",
 	}
 	for _, rel := range []string{
 		metadata.FirecrackerBin,
 		metadata.SSHKeyPath,
 		metadata.NamegenPath,
 		metadata.CustomizeScript,
+		metadata.VSockPingHelperPath,
 		metadata.DefaultPackages,
 		metadata.DefaultRootfs,
 		metadata.DefaultKernel,