docs: add plugin-oriented reusable design / 文件補上插件化與多 agent 重用設計

README.md

@@ -28,6 +28,8 @@
 - 將使用者選擇映射到對話狀態
 - 保留使用者直接打字而不按按鈕的能力
 
+此外，這個專案從一開始就要考慮**插件化**，讓未來不只一個 assistant 可以使用，而是多個 agent / bot / assistant runtime 都可以共用同一套 reply-end-controls 能力。
+
 ### 專案目標平台
 
 這個專案目前 **固定只做 Telegram**。
@@ -160,6 +162,8 @@ This repository is only for the reply-end control feature, including:
 - mapping user choice into conversation state
 - preserving the ability to type freely instead of pressing a button
 
+From the beginning, this project should also be designed as a **reusable plugin-style component** so multiple agents or assistant runtimes can adopt the same reply-end control behavior instead of implementing it independently.
+
 ### Target platform
 
 This project is **Telegram-only** for now.