Prefix default train output dir with current date

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

giant/cli.py

@@ -1,5 +1,5 @@
 from collections import Counter
-from datetime import datetime, timezone
+from datetime import date, datetime, timezone
 from enum import Enum
 from pathlib import Path
 from typing import Optional
@@ -245,7 +245,8 @@ def train(
         )
 
     out_dir = out or Path(
-        f"checkpoints/{t['mode']}"
+        f"checkpoints/{date.today().strftime('%Y%m%d')}"
+        f"_{t['mode']}"
         f"_h{m['hidden_dim']}"
         f"_b{m['n_blocks']}"
         f"_e{m['emb_dim']}"