ci: exclude sponsorkit cache from sponsor updates

[CodeDredd]

The first sponsors workflow run committed sponsorkit's .cache.json alongside the images — it changes on every run and doesn't belong in the repo. Removed from git, ignored, and the workflow now stages only the three image/data files.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.34%. Comparing base (05a9386) to head (cab0404).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2050      +/-   ##
==========================================
+ Coverage   99.24%   99.34%   +0.09%     
==========================================
  Files          83       83              
  Lines        3185     3185              
  Branches      562      562              
==========================================
+ Hits         3161     3164       +3     
+ Misses         17       15       -2     
+ Partials        7        6       -1     

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