helper + github actions for QA?

on push and cron those could at least verify that every annexed file is still available from our S3

I think we need to establish some helper to given a dandiset execute necessary tests + create a github action so that backups2datalad.py then should instantiate workflows invoking that action which "installs" all necessary for helper dependencies and runs the tests and reports on success/failure.

Start by reading backups2datalad.py and map how a dandiset is selected and how annexed files are currently checked. Define the helper and GitHub Actions entry points around the stated push/cron workflow, dependency installation, S3 availability checks, and success/failure reporting; done means those checks run reproducibly for a dandiset.

Written by the indexing model from the issue text.