feat: flip clawbench-batch default --cases-suite to v2

V2 is the published / supported corpus across claw-bench.com, the README
6-tab leaderboard, the TIGER-Lab/ClawBenchV2Trace HF dataset, and the
reproduce CLI's baseline table. Default the batch runner to v2 so any new
user invoking `clawbench-batch` without --cases-suite lands on the same
corpus that matches our scoring + traces.

V1 still works (`--cases-suite v1`) and the V1 NAIL-Group/ClawBenchV1Trace
dataset is unchanged.

Author: reacher-z

src/clawbench/runner/batch.py

@@ -47,7 +47,7 @@ def detect_engine() -> str:
     "v2": "test-cases/v2",
     "v1-lite": "test-cases/v1-lite",
 }
-DEFAULT_CASES_SUITE = "v1"
+DEFAULT_CASES_SUITE = "v2"
 
 
 def load_models_yaml() -> dict: