feat(key_points): add predefined HAND skeleton for 21-keypoint hand landmarks

[SkalskiP]

Summary

• Add Skeleton.HAND to the predefined skeleton enum in key_points/skeletons.py
• Define the standard 21-keypoint hand topology (MediaPipe / COCO-WholeBody): wrist plus 4 joints per finger, 20 edges total
• Enable automatic skeleton lookup for hand keypoints via SKELETONS_BY_VERTEX_COUNT[21] in EdgeAnnotator

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81%. Comparing base (9251893) to head (3fb968a).

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #2349   +/-   ##
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  Coverage       81%     81%           
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  Files           66      66           
  Lines         9077    9078    +1     
=======================================
+ Hits          7377    7378    +1     
  Misses        1700    1700           

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