Remove SkyPilot backend and docs

[corbt]

Summary

• remove SkyPilot backend package, scripts, configs, and extras
• update docs/tutorials/examples to use Local/Serverless workflows
• convert Backend to a Protocol and adjust backend typing

Testing

• MAX_STEPS=1 MIN_BATCH_SIZE=1 NUM_ROLLOUT_WORKERS=1 ROLLOUTS_PER_SCENARIO=2 EVAL_EVERY_N_STEPS=1 EVAL_STEP_0=1 REQUEST_TIMEOUT=30 uv run --extra tinker python -m art.pipeline_trainer.binary_prefix_tool_pipeline

References

• Refs: RFC: Deprecating SkypilotBackend #510