Add dataset access control for model configurations

[ericc59]

Summary

• Add datasets field to models.yml for controlling which datasets a model can access
• Default to ["public-v1", "public-v2"] only; private datasets require explicit opt-in
• Add get_allowed_datasets() and is_dataset_allowed() utilities in task_utils.py
• Add cli/check_dataset.py for validating dataset access from shell scripts

Dataset Configuration

Models without a datasets field default to public datasets only:

datasets: ["public-v1", "public-v2"]  # DEFAULT if not specified

To allow private datasets, add explicit configuration:

datasets: ["public-v1", "public-v2", "private-v1", "private-v2"]

Test plan

• Verify cli/check_dataset.py returns exit code 0 for allowed datasets
• Verify cli/check_dataset.py returns exit code 1 for disallowed datasets
• Verify models without datasets field default to public only