fix(pool): use WAA native task distribution with --worker_id/--num_workers

- Fixed task distribution: WAA ignores --start_idx/--num_tasks, use native
  --worker_id and --num_workers parameters instead
- Worker 0 gets tasks 0, N, 2N... Worker 1 gets tasks 1, N+1, 2N+1...
- Use vanilla windowsarena/winarena image with correct IP (20.20.20.21)
- Add container reuse check (skip restart if already running)
- Pass API key via env var instead of config file
- Fix QMP port exposure (7200) for QEMU control
- Store Windows disk on /mnt for 300GB temp storage (D8ds_v5)

Tested: 2-worker pool running 4 tasks in parallel successfully

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

Author: abrichr