Implement a way to prevent sending sensitive information to remote APIs

[1over137]

I would like to use both local (ollama) and remote APIs (Claude), but I'm wary of accidentally sending my private notes to remote APIs for training or analytics. It would be nice if there is a way to block accidentally invoking remote APIs in some files.

There are a few ways this can be done. We can add a variable to block org-mode buffers by default for remote APIs and whitelist each file with a file property, or we can make the file-level property #+GPTEL_NO_REMOTE: t that would prevent sending the text, but I don't know enough about org-mode to know if that would work with narrowed buffers.