Draft: add minimal ask_user tool shape

[tunahorse]

This is a draft PR to show the shape of a minimal ask_user tool integration.

Scope in this draft:

• add AskUserRequest / AskUserSender types
• add the built-in ask_user tool
• add a small injected tool registry so built-in tools can be attached without changing provider handlers
• wire ask_user_channel(...) into Looper and LooperStream

This is meant to keep the patch small and make the feature's shape easy to review first.

I got an example working here https://github.com/alchemiststudiosDOTai/looper-rs/tree/asktool