Fix default log file extension to .json

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

collect.py

@@ -417,7 +417,7 @@ def run(args: argparse.Namespace) -> None:
         logs_dir = Path("logs")
         logs_dir.mkdir(exist_ok=True)
         stamp = datetime.now().strftime("%Y_%m_%d_%H_%M_%S")
-        out = logs_dir / f"{stamp}_{vin}.log"
+        out = logs_dir / f"{stamp}_{vin}.json"
 
     store = load_store(out, vin, args.interval)
     log.info(
@@ -498,7 +498,7 @@ def main() -> None:
         "-o", "--output",
         default=None,
         metavar="FILE",
-        help="Output file (default: logs/YYYY_MM_DD_HH_MM_SS_<VIN>.log)",
+        help="Output file (default: logs/YYYY_MM_DD_HH_MM_SS_<VIN>.json)",
     )
     p.add_argument(
         "--max-records",