docs(readme): add demo GIF (#160)

Author: meowgorithm

README.md

@@ -1,5 +1,7 @@
 # ⌬ OpenCode
 
+<p align="center"><img src="https://github.com/user-attachments/assets/9ae61ef6-70e5-4876-bc45-5bcb4e52c714" width="800"></p>
+
 > **⚠️ Early Development Notice:** This project is in early development and is not yet ready for production use. Features may change, break, or be incomplete. Use at your own risk.
 
 A powerful terminal-based AI assistant for developers, providing intelligent coding assistance directly in your terminal.