Support md export

[onmax]

I would like to run a CI and export the evalite output as md then pipe it to $GITHUB_OUTPUT, so the CI action shows the results of the evals.

Can we add support for it?

I can open a PR if you are interested :)

[mattpocock]

@onmax I'm not sure I'd yet be interested in supporting this yet. However, it would be pretty simple to write an adapter in user land or as a third party library to change the exported JSON into Markdown. Once that's stable, we could talk about pulling it into main.

[onmax]

For GitHub Actions job summary, here's an inline bash/node script that converts the JSON export to markdown: onmax/mcp-starter@5c893b6