This PR adds a banner to communicate the deprecation of agent server
extensions in favor of the ACP registry:

<img width="600" height="1986" alt="Screenshot 2026-01-27 at 8  37@2x"
src="https://github.com/user-attachments/assets/8c1b658f-d170-4009-a93b-336b785f4be9"
/>

Release Notes:

- N/A