Use cursor official ACP

[Lenbok]

I've been using this a fair bit over the last day and it generally works well.

Cursor has some extension points, and at least for _cursor/update_todos this caused the agent session to hang. I've implemented an error response to unknown messages, and this allows the session to proceed. It seems like an appropriate response in general. We can decide whether to support their custom extension points later.

Fixes: #386

Checklist

• [ x] I agree to communicate (PR description and comments) with the author myself (not AI-generated).
• I've reviewed all code in PR myself and will vouch for its quality.
• I've read and followed the Contributing guidelines.
• I've filed a feature request/discussion for a new feature.
• I've added tests where applicable.
• I've updated documentation where necessary.
• I've run M-x checkdoc and M-x byte-compile-file.

[Lenbok]

An example traffic session showing the _cursor/update_todos, the error response, and the session continuing:

agent-shell-cursor-update-todos.txt