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/daemon/imagebuild_test.go

@@ -26,6 +26,11 @@ func TestBuildProvisionScriptInstallsDefaultTools(t *testing.T) {
 		"set -g @continuum-restore 'off'",
 		"set -g @resurrect-dir '/root/.tmux/resurrect'",
 		"run '~/.tmux/plugins/tpm/tpm'",
+		"cat > /etc/modules-load.d/banger-vsock.conf <<'EOF'",
+		"vmw_vsock_virtio_transport",
+		"cat > /etc/systemd/system/banger-vsock-pingd.service <<'EOF'",
+		"ExecStart=/usr/local/bin/banger-vsock-pingd",
+		"systemctl enable --now banger-vsock-pingd.service || true",
 		"rm -f /root/get-docker /root/get-docker.sh /tmp/get-docker /tmp/get-docker.sh",
 	} {
 		if !strings.Contains(script, snippet) {