Use a separate PR workflow with gates

[christophfroehlich]

Description

Should reduce github runner usage if any of the linter gates fail.
Drawback: one more file to maintain.

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GPT Codex 5.3

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.53%. Comparing base (4bcce09) to head (5194e1f).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3474      +/-   ##
==========================================
+ Coverage   89.51%   89.53%   +0.01%     
==========================================
  Files         164      164              
  Lines       21265    21265              
  Branches     1658     1658              
==========================================
+ Hits        19036    19040       +4     
+ Misses       1530     1526       -4     
  Partials      699      699              

Flag	Coverage Δ
unittests	89.53% <ø> (+0.01%)	⬆️

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see 2 files with indirect coverage changes

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