Add runtime environment checks for Podman in panelctl script

Co-authored-by: Copilot <copilot@github.com>

README.md

@@ -29,6 +29,8 @@ Notes:
 - The default compose file uses traefik/whoami for smoke testing.
 - Edit each generated compose.yaml before production use.
 - App names must be lowercase slugs.
+- If deploy reports XDG_RUNTIME_DIR missing, enable lingering for the runtime user:
+   `sudo loginctl enable-linger reudy`
 
 API service:
 - Nix now runs panel-api as a systemd service on 127.0.0.1:9911.