feat: Make stub proxy target derive from MCP_PORT env var

stub/src/main.rs

@@ -80,7 +80,12 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
             let _ = shutdown_tx.send(()).await;
         });
 
-        let target_url = cli.target;
+        let target_url = if cli.target != "http://localhost:3000" {
+            cli.target
+        } else {
+            let port = std::env::var("MCP_PORT").unwrap_or_else(|_| "3000".to_string());
+            format!("http://127.0.0.1:{}", port)
+        };
         let ws_url = target_url.replace("http://", "ws://").replace("https://", "wss://");
         let ws_url = format!("{}/ws?client=proxy", ws_url);
         let msg_rx = Arc::new(tokio::sync::Mutex::new(msg_rx));