Docs: migrate Coveralls badge to Codecov

[ohmyfelix]

What changed

• switched the README coverage badge from Coveralls to Codecov to match the current Contributte migration pattern
• verified there are no remaining Coveralls references in the tracked project files and that .github/workflows/coverage.yml already uses the shared coverage workflow

Why

• this finishes the remaining Coveralls-to-Codecov migration work for contributte/console-extra
• keeps the repository aligned with Migrate remaining Coveralls usage to Codecov across contributte repos contributte#72

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 47.51%. Comparing base (09e14dc) to head (0572b3a).
⚠️ Report is 10 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #54      +/-   ##
==========================================
+ Coverage   43.44%   47.51%   +4.07%     
==========================================
  Files          24       24              
  Lines         557      524      -33     
==========================================
+ Hits          242      249       +7     
+ Misses        315      275      -40     

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