I want to be able to keep some files private

For example, PDFs of references. I am not allowed to post those publicly but I do want to back them up and view them on the website.

This could be done pretty easily for DVC files. Ones publicly available on GitHub not so much.

Another option:

```sh
calkit add my-path --private
```

This could work similar to DVC zip storage, where we sync private files into a private git repo in `.calkit/local`.

Start at the `calkit add my-path --private` entry point and compare the proposal with the existing DVC zip-storage behavior. Trace how private files could be synchronized into a private Git repository under `.calkit/local`. Done should include a defined private-file workflow that supports backing up and viewing files without making them public.

Written by the indexing model from the issue text.