what is the correct forking behavior with respect to files?

Currently, forking does _not_ copy files. That seems like perhaps the right approach, but I can see how this might cause confusion. At any rate, the notebook client needs to have some logic to clear the `files` part of the store when a notebook is forked.

Start by tracing the notebook client's fork flow and how the store's `files` section is handled. Resolve whether forked notebooks should retain files; if they should not, clear that section during forking and verify the resulting store behavior with the project's existing checks.

Written by the indexing model from the issue text.