Remove IMPROVEMENT_SUMMARY.md - content already documented elsewhere

The implementation details are already documented in:
- The actual merged code in lib/react_on_rails/smart_error.rb
- The user guide at docs/guides/improved-error-messages.md
- The original PR #1834 commit messages

No need for a separate summary document.

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: justin808