#228: R9: Improve type safety in config.ts validateConfigTypes

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Summary

This PR improves type safety in validateConfigTypes() by introducing typed helper functions to encapsulate unsafe type casts.

Changes Made

• Added deleteField() helper function to encapsulate delete operations with type-safe interface
• Added setField() helper function to encapsulate assignments with type-safe interface
• Replaced 3 instances of delete (validated as Record<string, unknown>)[field] with deleteField() calls
• Replaced 1 instance of (validated as Record<string, unknown>)[field] = value with setField() call

Verification

✅ TypeScript compilation passes (npm run typecheck)
✅ All 398 tests pass (npm test)
✅ Linting and formatting checks pass (npm run lint)
✅ Build succeeds (npm run build)

Impact

• Risk Level: Very Low
• Behavior Change: None - purely internal type handling improvement
• Test Coverage: All existing comprehensive tests in config.test.ts continue to pass

The unsafe as Record<string, unknown> cast is now confined to two helper functions, making the rest of the validation code more type-safe and easier to read.

Closes #228

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