Align changelog PR check with push versioning

[anth-volk]

Summary

The PR changelog gate was using towncrier check --compare-with origin/master, which allowed PRs with no new fragment to pass in some branch histories while the push workflow still failed at bump_version.py with No changelog fragments found.

This switches the PR check to the same effective contract the push workflow enforces: a PR must contain at least one real fragment file in changelog.d/.

Changes

• replace the towncrier check --compare-with origin/master PR gate with a direct fragment-presence check
• keep .gitkeep excluded
• add a changelog fragment for this workflow fix

Validation

• verified the new PR check logic sees a fragment in changelog.d/
• ran uv run python .github/bump_version.py to confirm the fragment is consumable by the push-time versioning script

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.00%. Comparing base (08ba60c) to head (272d2ad).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3505   +/-   ##
=======================================
  Coverage   77.00%   77.00%           
=======================================
  Files          63       63           
  Lines        3418     3418           
  Branches      617      617           
=======================================
  Hits         2632     2632           
  Misses        612      612           
  Partials      174      174           

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