Stabilize multi-agent feature flag (#14622)

- make multi_agent stable and enabled by default
- update feature and tool-spec coverage to match the new default

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Co-authored-by: Codex <noreply@openai.com>

codex-rs/tui/src/app.rs

@@ -5074,6 +5074,7 @@ mod tests {
     #[tokio::test]
     async fn open_agent_picker_prompts_to_enable_multi_agent_when_disabled() -> Result<()> {
         let (mut app, mut app_event_rx, _op_rx) = make_test_app_with_channels().await;
+        let _ = app.config.features.disable(Feature::Collab);
 
         app.open_agent_picker().await;
         app.chat_widget