[Feature]: 功能请求：能否给插件API增加一个system_prompt的权限

功能请求：
能否给插件API增加一个system_prompt的权限，公司做插件互动界面有公司的提示词，不想提交会话后在qwenpaw的会话界面被用户看到。

No file or test is named. Start by locating the plugin API and the conversation submission and display flow, then determine how permissions are represented. Done means a plugin can use a system_prompt permission so the company's prompt is not exposed in QwenPaw's conversation UI after submission, while existing plugin interactions continue to work.

Scritto dal modello di indicizzazione a partire dal testo della issue.