How should we handle external requirements?

[skissue]

This is a three-in-one for the "types" of external requirements as I see them:

• External Emacs packages (e.g. mu4e).
• External system binaries (e.g. ripgrep).
• External authentication (e.g. Brave Search API).

I'd like to agree on a sensible framework for handling these. The main issue is that having standard checks within the tool bodies may not be a great UX, because those errors will only be caught at tool-call-time, and thus exposed to the model instead of the user.

However, since we simply expose tools as symbols with the variable value containing the spec, we can't check these requirements when adding a tool either.

Furthermore, there is a difference between synchronous and asynchronous tools here: errors within a synchronous tool will be forwarded to the model, whereas errors within an asynchronous tool will be bubbled up to the user, and the tool call will hang forever.

How should we handle this?