feat: stream exec stdout events (#1786)

## Summary
- stream command stdout as `ExecCommandStdout` events
- forward streamed stdout to clients and ignore in human output
processor
- adjust call sites for new streaming API

codex-rs/core/src/shell.rs

@@ -220,6 +220,7 @@ mod tests {
                 Arc::new(Notify::new()),
                 &SandboxPolicy::DangerFullAccess,
                 &None,
+                None,
             )
             .await
             .unwrap();