docs(readme): replace the star-history chart with a live fan-out clip

GitHub restricted third-party access to star data, so the star-history
image at the bottom of the README no longer renders. Replace it with a
9-second clip of Codewhale fanning out three read-only scout subagents
(assets/fanout.gif, 960px, 2.3 MB).

(cherry picked from commit f22bade09b)

README.md

@@ -133,4 +133,4 @@ terminal-agent experience.
 [MIT](LICENSE). An independent community project, not affiliated with any model
 provider.
 
-[![Star History Chart](https://api.star-history.com/chart?repos=Hmbown/CodeWhale&type=date&legend=top-left)](https://www.star-history.com/?repos=Hmbown%2FCodeWhale&type=date)
+![Codewhale fanning out three read-only scout subagents in a terminal](assets/fanout.gif)