fix: restore stdout output for standalone --headless mode

RenderMode::Silent was incorrectly applied to --headless in addition to
--acp-server. Standalone headless should still display the LLM response;
only ACP mode requires stdout purity for JSON-RPC. The acp server's
run_prompt_turn already sets Silent before each prompt call.

src/main.rs

@@ -236,7 +236,7 @@ async fn main() -> Result<()> {
         }
     }
 
-    if cli.headless || cli.acp_server {
+    if cli.acp_server {
         ctx.render_mode = RenderMode::Silent;
     }