feat(hpc): support run_dir staging and update docs

- Add run_dir and checkpoint_frequency to PPOArgs
- Update train.py to use run_dir for SummaryBoard, model saving, and loss plots
- Create configs/production_training.yaml for HPC production runs
- Update HPC.md with run_dir staging strategy details

src/brittle_star_project/dataclasses/PPOArgs.py

@@ -13,6 +13,12 @@ class PPOArgs:
     # the name of this experiment
     exp_name: str = "brittle_star_ppo"
 
+    # the directory to save the experiment results
+    run_dir: str | None = None
+
+    # how often to save checkpoints (0 to disable)
+    checkpoint_frequency: int = 0
+
     # seed of the experiment
     seed: int = 1