chore: Move SKILL.md to subdirectory

[Oliver Borchert (borchero)]

Motivation

The current design causes the entire repo to be cloned to .agent/skills when using npx skills add Quantco/dataframely.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (eccd34c) to head (2bce09c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #312   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           56        56           
  Lines         3233      3233           
=========================================
  Hits          3233      3233           

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