refactor: 将多个 fmt.Println 日志替换为 log.Printf，以统一日志输出格式并提升可读性

handler/headmaster/headmaster.go

@@ -111,7 +111,7 @@ func ask(from string, question string) (reply string) {
 	)
 
 	if err != nil {
-		fmt.Printf("ChatCompletion error: %v\n", err)
+		log.Printf("ChatCompletion error: %v\n", err)
 		return
 	}
 	AppendReplyToHistory(from, resp.Choices[0].Message.Content)