Docs: switch coverage badge to Codecov

[ohmyfelix]

What changed

• replaces the README coverage badge link from Coveralls to Codecov
• keeps the repo aligned with the existing Codecov-based coverage workflow

Why

• completes the Coveralls-to-Codecov migration for contributte/dev
• removes the last public Coveralls reference tracked in Migrate remaining Coveralls usage to Codecov across contributte repos contributte#72

Ref: contributte/contributte#72

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (master@a2770c8). Learn more about missing BASE report.

Additional details and impacted files

@@           Coverage Diff            @@
##             master     #13   +/-   ##
========================================
  Coverage          ?   3.80%           
========================================
  Files             ?       2           
  Lines             ?     105           
  Branches          ?       0           
========================================
  Hits              ?       4           
  Misses            ?     101           
  Partials          ?       0           

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