docs(readme): rename hero icon to bust GitHub's raw-asset CDN cache

The previous commit replaced icon.png on disk with the rust v2.5
artwork, but GitHub's raw-asset CDN was still serving the cached
purple PNG to README viewers (~5 min TTL — but in practice longer
under sustained traffic). Renaming the asset forces a fresh fetch
on every README render, which is the reliable cache-bust.

icon-v2.5.png is bit-identical to the prior icon.png (md5 match
against the Mac app icon set's 512x512). The version in the
filename is intentional — when v2.6 ships with a different icon,
we'll cycle to icon-v2.6.png and the same cache-bust applies.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -1,5 +1,5 @@
 <p align="center">
-  <img src="icon.png" width="128" height="128" alt="Scarf app icon">
+  <img src="icon-v2.5.png" width="128" height="128" alt="Scarf app icon">
 </p>
 
 <h1 align="center">Scarf</h1>