docs: add Korean README (README.ko.md)

[amondnet]

Summary

• Create README.ko.md with full Korean translation
• Add language selector to both README files

Test plan

• Verify Korean README renders correctly
• Verify language selector links work

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 33.09%. Comparing base (9a691e1) to head (b812b27).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #25   +/-   ##
=======================================
  Coverage   33.09%   33.09%           
=======================================
  Files          13       13           
  Lines        2798     2798           
=======================================
  Hits          926      926           
  Misses       1872     1872           

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