Revert "Refresh README with Cotabby brand banner"

[FuJacob]

Reverts #784

Greptile Summary

This PR reverts the README brand banner update. The main changes are:

• Removes the old cotabby-banner.png README asset.
• Points the README header image back to .github/assets/readme/logo.png.
• Adds a centered title and short product tagline under the logo.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.
• The README now points to an existing repo asset with matching path and case.
• No tracked files still reference the deleted banner image.

Important Files Changed

Filename	Overview
README.md	Replaces the wide banner with the existing logo asset and adds centered title/tagline markup.
.github/assets/readme/cotabby-banner.png	Deletes the old banner image that is no longer referenced by tracked files.

_{Reviews (1): Last reviewed commit: "Revert "Refresh README with Cotabby bran..." | Re-trigger Greptile}