Address PR feedback: revert notebook reformatting noise, apply targeted renames, and document CodeEvaluator name change

Author: gigistark-google

CHANGELOG.md

@@ -10,7 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added
 
 - Added `SystemEvaluator` as the preferred name for deterministic/code-defined metrics.
-- Kept `CodeEvaluator` as a backward-compatible alias (deprecated but supported).
+- Kept `CodeEvaluator` as a backward-compatible alias (deprecated but supported). Note that calling `CodeEvaluator()` now emits `evaluator_name="system_evaluator"`.
 
 ## [0.3.2] - 2026-05-22