docs: remove stale IMPLEMENTATION_PLAN.md

[Zethson]

Removes IMPLEMENTATION_PLAN.md, which was a one-off planning doc no longer referenced anywhere in the repo.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.54%. Comparing base (a8273c8) to head (901a68d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #37   +/-   ##
=======================================
  Coverage   95.54%   95.54%           
=======================================
  Files           6        6           
  Lines         359      359           
=======================================
  Hits          343      343           
  Misses         16       16           

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