docs(readme): replace floating hero with full-bleed brand banners (#103123)

The README led with a transparent mascot cutout floating on the GitHub
background. Swap in generated full-bleed banners (ink starfield dark
variant, warm paper light variant) with the wordmark, tagline, and
one-liner baked in, and drop the now-redundant EXFOLIATE text line.

README.md

@@ -2,15 +2,11 @@
 
 <p align="center">
     <picture>
-        <source media="(prefers-color-scheme: light)" srcset="https://raw.githubusercontent.com/openclaw/openclaw/main/docs/assets/openclaw-hero-light.png">
-        <img src="https://raw.githubusercontent.com/openclaw/openclaw/main/docs/assets/openclaw-hero-dark.png" alt="OpenClaw" width="500">
+        <source media="(prefers-color-scheme: light)" srcset="https://raw.githubusercontent.com/openclaw/openclaw/main/docs/assets/openclaw-banner-light.png">
+        <img src="https://raw.githubusercontent.com/openclaw/openclaw/main/docs/assets/openclaw-banner-dark.png" alt="OpenClaw — EXFOLIATE! EXFOLIATE! Your personal AI assistant, running on your own devices.">
     </picture>
 </p>
 
-<p align="center">
-  <strong>EXFOLIATE! EXFOLIATE!</strong>
-</p>
-
 <p align="center">
   <a href="https://github.com/openclaw/openclaw/actions/workflows/ci.yml?branch=main"><img src="https://img.shields.io/github/actions/workflow/status/openclaw/openclaw/ci.yml?branch=main&style=for-the-badge" alt="CI status"></a>
   <a href="https://github.com/openclaw/openclaw/releases"><img src="https://img.shields.io/github/v/release/openclaw/openclaw?include_prereleases&style=for-the-badge" alt="GitHub release"></a>