#410 ARTIFACT mode in code_versioning doesn't auto-load from code_artifacts table

Review Gap 3 incomplete: CodeVersionMode.ARTIFACT is defined but python_task.py doesn't auto-load code from code_artifacts table when mismatch detected. Currently just logs warning. Should: (1) detect mismatch, (2) fetch source via get_code_artifact(conn, stored_hash), (3) exec() that source instead of importing from disk. This completes the 'Code as Data' recommendation.