[CI] Bouncer

[novusnota]

Check the size of the PR from a fork (additions, deletions)

Let's set the boundary at $\mathrm{additions} + \mathrm{deletions} > 1000$. Don't do it silently — write a good explanatory message that should describe what and why, and also outline next steps.

Instead of closing those that are too big we might just fail the CI instead. Additionally, let's only look for additions and reduce the threshold to 500 for the forks and keep 1000 for direct branches.