docs: refresh README hero banner

[minpeter]

Summary

• replace the existing README hero banner with the new OpenMicro artwork
• keep the README markup and asset path unchanged
• reduce the banner file size from about 1.4 MB to 800 KB

Test plan

• Confirm the committed PNG matches the supplied source image
• Confirm the diff changes only assets/open-micro-banner.png
• Visually inspect the 5120×1280 banner

[minpeter]

@stephenleo If you don't like it, feel free to close it

Previous works

• https://github.com/Soju06/codex-lb [PR]
• https://github.com/minpeter/pss-runtime
• https://github.com/minpeter/opensearch