feat(evaluate): update CLI for sharded checkpoint system

Update evaluate.ts to support sharded checkpoints:

- Read worker identity from GRAPHBOX_WORKER_INDEX and GRAPHBOX_TOTAL_WORKERS
- Use worker-specific checkpoint path when in worker mode
- Add FileStorage.findShards() call to discover worker checkpoints
- Add checkpoint merge phase after parallel execution completes
- Direct CheckpointManager instantiation with DI (no factory)

The main process now aggregates worker checkpoint shards into a single
checkpoint.json after all workers complete.

Author: Mearman