Fixes#3031
When a subagent completes and announces its result back to the main
agent, the main agent's pending tasks were being lost due to a race
condition between the followup queue drain and the announce flow.
Changes:
- Add persistence layer for followup queues (similar to subagent runs)
- Implement 30-second grace period before deleting empty queues
- Persist queue state after enqueue/dequeue operations
- Track emptyAt timestamp to prevent premature deletion
- Add periodic cleanup for expired empty queues
This ensures that when a subagent completes and triggers a new main
agent invocation, any pending tasks in the followup queue are
preserved and restored from disk.
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
