fix: pin CI status badge to default branch

[lirantal]

The CI status badge in the README currently points to the unpinned workflow URL, which can show the status of any branch (e.g. PR runs) rather than the default branch. Pinning the badge to the repository default branch so it consistently reflects mainline CI.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.70%. Comparing base (f8be598) to head (28944e0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #119   +/-   ##
=======================================
  Coverage   89.70%   89.70%           
=======================================
  Files           5        5           
  Lines         204      204           
  Branches       63       63           
=======================================
  Hits          183      183           
  Misses         20       20           
  Partials        1        1           

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