Fix issue with GPT pointing to files at sandbox:

[simonmesmith]

For example, when GPT refers to an image, it sometimes does <img src="sandbox:image_name.png">. I think it does this because it's trained to do so for environments like ChatGPT. This causes issues when it does things like write HTML pointing to images at nonexistent locations.

[simonmesmith]

Maybe we need to include the output path as part of the system message.