Add queue management system for task queue architecture (Phase 1):
- Add QUEUE_FILE and POLL_INTERVAL constants
- Add init_queue() to initialize queue.json if missing
- Add cmd_queue with subcommands:
- list: Show queue status with counts per tier
- enqueue <tier> <msg>: Add task to queue
- dequeue [tier]: Remove and return next task (priority order)
- clear: Clear all queued tasks
Queue tiers:
- dev_followups (highest priority)
- user_interrupts (medium)
- background (lowest)
This enables the autonomous queue-based architecture where PM agent
continuously polls the queue and assigns work to dev agents.
Part of #49
0f66de2929