Support Agent Client Protocol (ACP)

If you support Agent Client Protocol (ACP), then I can just use an ACP compatible agent such as "Google Gemini CLI", and I don't need to specify an API key.

https://geminicli.com/docs/cli/acp-mode/

also, note how Goose supports it.

<img width="876" height="1011" alt="Image" src="https://github.com/user-attachments/assets/ba6ada28-2eac-4780-883a-aae8bc43b9fc" />