Bare run in a terminal prints the help screen; add -h/--help

A zero-argument invocation now shows the usage text when stdout is a
console (double-clicked exe, interactive shell). Without a terminal —
Docker entrypoint, services, pipes — a bare invocation still starts the
proxy, so the container image and service behavior are unchanged.

README.md

@@ -91,9 +91,13 @@ override file values. Invalid values fail at startup.
 
 ```sh
 go build ./cmd/gpu-turnstile
-./gpu-turnstile
+OLLAMA_URL=http://127.0.0.1:11435 COMFY_URL=http://127.0.0.1:8189 ./gpu-turnstile
 ```
 
+Running the binary with no arguments in a terminal prints the help screen
+(same as `-h`/`--help`); without a terminal (services, containers) a bare
+invocation starts the proxy.
+
 ### Run natively on Windows (current primary deployment)
 
 Download `gpu-turnstile.exe` from a release and install it as a Windows