docs: replace banner.png with hero.png in README

The previous PR (#88) accidentally added hero.png alongside banner.png
instead of replacing it. Now: hero.png is the single top image, banner.png
removed (it had no other references in the repo).

Author: shreyas-lyzr

README.md

@@ -1,9 +1,5 @@
 <p align="center">
-  <img src="banner.png" alt="gitagent banner" width="700" />
-</p>
-
-<p align="center">
-  <img src="hero.png" alt="OpenGAP — GitAgentProtocol (Open GAP)" width="640" />
+  <img src="hero.png" alt="OpenGAP — GitAgentProtocol (Open GAP)" width="700" />
 </p>
 
 # OpenGAP | your repository becomes your agent