fix(coding-agent): forward RpcClient subprocess stderr to parent in real-time

fixes #2805

packages/coding-agent/src/modes/rpc/rpc-client.ts

@@ -92,6 +92,7 @@ export class RpcClient {
 		// Collect stderr for debugging
 		this.process.stderr?.on("data", (data) => {
 			this.stderr += data.toString();
+			process.stderr.write(data);
 		});
 
 		// Set up strict JSONL reader for stdout.