Core changes:
- Add internal/worker package: Handler registry, BackoffStrategy interface,
  FixedBackoff, ExponentialBackoff, Worker (single consumer), Pool (multi-queue
  multi-worker orchestrator) with graceful shutdown and per-worker state snapshots
- Fix retry bug in both backends: Nack previously set non-final failures to
  "failed" status, which Reserve never picks up (it only selects "pending" jobs).
  Non-final failures now go back to "pending" with scheduled_at = now + retryDelay
- Extend Backend.Nack signature to accept retryDelay time.Duration so the worker
  can pass its computed backoff to the backend atomically
- Update all backend tests to use the new Nack signature and assert "pending"
  status (not "failed") for retryable jobs
- Mark Phase 2 complete in PLAN.md

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>