Tasks should default to ACP instead of AgentAPI #21133

ACP is a better protocol for communicating with agents programmatically. We should make this the default way to talk to agents from the Tasks UI and API.

Things to research:

• Do we entirely rip out AgentAPI, or only use the UI elements, or only use it for agents not compatible with ACP?
• What is a solid UI we can use in the sidebar for communicating with the agents? Should it still be a coder_app?