Implement "preview" comments on PRs

[holly-cummins]

It'd be great to have CI pop a comment with a few sample graphs onto PRs so we can eyeball before merging.

https://stackoverflow.com/questions/41581151/how-to-upload-an-image-to-use-in-issue-comments-via-github-api discusses how to do this, and the two options are to make a branch, or to reverse engineer the github API.

[holly-cummins]

OK, so that didn't work. The artifacts get uploaded as zips, even for a single file (https://github.com/actions/upload-artifact?tab=readme-ov-file#zip-archives), so it looks like I'll have to create a branch for the images. How annoying.

[edeandrea]

You could add a download step then manually unzip them. But yes the upload-artifact action will zip everything up

[holly-cummins]

You could add a download step then manually unzip them. But yes the upload-artifact action will zip everything up

The problem with that is that what I need is a URL. So having the artifacts unzipped on the machine running the workflow doesn't help. Or rather, it's a useful step, but then I need to upload them to somewhere, like a branch, or a temporary storage service.

[holly-cummins]

Halfway there, the no-op case is working. :)

[holly-cummins]

Victoreeeee!

#117 (comment)

But that was annoyingly time-consuming to get right.