[Feature Request] report PRs that bring down type coverage

[azjezz]

A recent PR I made to PSL, lowered the type coverage from 100% to 99%, and I didn't notice until after it got merged and I saw the badge. while the fix was pretty easy ( 00602d71 ), I would have rather the CI failed in the PR or at least shepherd bot commented to alert about the type coverage decrease.

[paulshryock]

I like the idea of a coverage threshold which would fail builds if Psalm coverage was below the threshold... something like vendor/bin/psalm --coverage --coverageThreshold=100... but that seems like more of a job for Psalm than for the Shepherd package. 🤷