feat(cli): add multi-VM pool commands for parallel WAA evaluation

Add pool-create, pool-wait, and pool-run commands for running WAA
benchmarks across multiple VMs in parallel:

- pool-create --workers N: Create N VMs with Docker and WAA image
  - Parallel VM creation using ThreadPoolExecutor
  - Auto-selects available region and VM size
  - Configures Docker with /mnt storage
  - Registers pool for tracking

- pool-wait: Wait for WAA to be ready on all workers
  - Starts WAA containers on each worker
  - Polls /probe endpoint until ready
  - Configurable timeout

- pool-run --tasks N: Distribute tasks across pool
  - Round-robin task distribution
  - Parallel execution on all workers
  - Progress tracking in registry

This enables ~5x faster benchmark completion with 5 workers,
or full 154-task evaluation in ~10min with 10+ workers.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: abrichr