Add a code coverage report pre and post PR

We should add a coverage report pre-/post-PR, in whichever form, to make sure we're not reducing code coverage (or at least not by too much).

No files, tests, or CI entry points are named in the issue. First survey the repository and its pull-request workflow to identify where coverage is produced and reported; the work is done when pre- and post-PR coverage can be compared and an unacceptable decrease is detected.

Written by the indexing model from the issue text.