Change default OpenAI model (gpt-4.1 -> gpt-5)

Ref: https://openai.com/index/introducing-gpt-5/

src/agent/provider/openai/config.rs

@@ -80,7 +80,7 @@ impl Default for TextGenerationConfig {
 }
 
 fn default_text_model_id() -> String {
-    "gpt-4.1".to_owned()
+    "gpt-5".to_owned()
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]