fix: suppress subagent error and warning output to stderr/stdout

src/config/agent.rs

@@ -623,6 +623,7 @@ impl Agent {
             vec!["_instructions".into(), "{}".into()],
             self.variable_envs(),
             Some(self.name().to_string()),
+            false,
         )?;
         match value {
             Some(v) => Ok(v),