feat(backend): add exclude option to pull_from_s3 for selective directory sync

[srthkdev]

• Add exclude parameter to Backend._experimental_pull_from_s3 to skip specified dirs
• Supported exclude values: checkpoints, logs, trajectories
• Update API to accept exclude list and forward it to backend method
• Enhance CLI endpoint to pass exclude option when pulling from S3
• Document exclude usage in method docstring with valid options listed
• Maintain backward compatibility with default None exclude behavior

solves #350