feat(config): add checkpoint frequency parameter

Add checkpoint_frequency to PPOArgs to enable periodic checkpoint
saving during training. Default: save every 100 iterations.

src/brittle_star_project/dataclasses/PPOArgs.py

@@ -26,7 +26,7 @@ class PPOArgs:
     wandb_project_name: str = "PPO-Modularity"
 
     # the entity (team) of wandb's project
-    wandb_entity: str | None = None
+    wandb_entity: str | None = "tdpeuter-ghent-university"
 
     # whether to capture videos of the agent performances (check out `videos` folder)
     capture_video: bool = False
@@ -34,6 +34,9 @@ class PPOArgs:
     # whether to save model into the `runs/{run_name}` folder
     save_model: bool = True
 
+    # checkpoint frequency (in iterations, 0 = no intermediate checkpoints)
+    checkpoint_frequency: int = 100
+
     # whether to upload the saved model to huggingface
     upload_model: bool = False