I want to be able to use Calkit in CI and see changes to artifacts in PRs

We could have a command at the end of a GitHub Actions workflow like:

```sh
calkit hub update-pr ${{ github.event.pull_request.number }}
```

Not sure what the best format would be. If we're talking about a LaTeX publication, we could create a LaTeX diff and upload the PDF. We could build a special page on calkit.io for comparing branch artifacts side-by-side and link to that. Might make sense under a "Changes" section.

See https://github.com/calkit/calkit/issues/451#issuecomment-3180520669 for more motivation

It may be good to also automatically create/update the workflow with:

```sh
calkit update actions-workflow --update-pr
```

Start with the proposed `calkit hub update-pr` command and the existing `calkit update actions-workflow --update-pr` entry point. Read issue #451 and its linked motivation, then determine the artifact comparison format and workflow behavior. Done means a decided approach for updating PRs with useful artifact changes, such as LaTeX diffs, uploaded PDFs, or a comparison page.

Written by the indexing model from the issue text.