chore: Add .cursor folder with rules

[GuillaumeRx]

This is an experiment with rules on common patterns in the monorepo. Most of it was written with Claude and personal guidance.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.39%. Comparing base (413290c) to head (bc9b6cc).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3825   +/-   ##
=======================================
  Coverage   98.39%   98.39%           
=======================================
  Files         430      430           
  Lines       12454    12454           
  Branches     1936     1936           
=======================================
  Hits        12254    12254           
  Misses        200      200           

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