Fix wheel build missing subpackages

[aaronspring]

Summary

• Replaces packages = ["xskillscore"] with [tool.setuptools.packages.find] auto-discovery
• The old config caused xskillscore.core, xskillscore.versioning, and xskillscore.tests to be omitted from built wheels, making installed packages non-functional
• Auto-discovery picks up all subpackages automatically, including any added in future

Closes #423

Test plan

• Verify python -m build --wheel produces a wheel containing xskillscore/core/, xskillscore/versioning/
• Existing test suite passes (pytest -n auto)

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.57%. Comparing base (eccc957) to head (0870f4b).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #448      +/-   ##
==========================================
+ Coverage   94.49%   94.57%   +0.08%     
==========================================
  Files          27       27              
  Lines        2836     2841       +5     
==========================================
+ Hits         2680     2687       +7     
+ Misses        156      154       -2     

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