dev: Add summary information to CI runs

[tsibley]

See commit messages for details.

See also GitHub Actions' documentation on step summaries.¹

¹ https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#adding-a-job-summary

Testing

• [x] 👀 the CI run for this PR; does it look ok?
• [x] Checks pass

[tsibley]

Hey cool

[tsibley]

Check it, more summarizing and addressing #13.

[tsibley]

Going to merge this since it's low-risk dev-only changes and works in CI (the main point) and for me locally. I can address any review comments or issues that arise in subsequent PRs.

[corneliusroemer]

Very nice ❤️