feat: update default (#4076)

Changes:
- Default model and docs now use gpt-5-codex. 
- Disables the GPT-5 Codex NUX by default.
- Keeps presets available for API key users.

codex-rs/mcp-server/tests/suite/set_default_model.rs

@@ -69,7 +69,7 @@ fn create_config_toml(codex_home: &Path) -> std::io::Result<()> {
     std::fs::write(
         config_toml,
         r#"
-model = "gpt-5"
+model = "gpt-5-codex"
 model_reasoning_effort = "medium"
 "#,
     )