chore: replace SVG banner with Anthropic-style PNG

Gemini-generated banner: warm cream background, coral accents,
clean flow diagram showing OpenAPI → 9 agent formats.

Author: koriyoshi2041

README.md

@@ -3,7 +3,7 @@
 <h1>Agentify</h1>
 <p><strong>Agent Interface Compiler</strong> — One command. Every agent speaks your product.</p>
 
-<img src="assets/banner.svg" alt="Agentify — OpenAPI to 9 agent interface formats" width="800">
+<img src="assets/banner.png" alt="Agentify — OpenAPI to 9 agent interface formats" width="800">
 
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