Add task auction lifecycle with full permission enforcement:
- TaskStore (SQLite) for tasks and bids CRUD, expiry, channel cancellation
- SwarmService with PostTask, BidOnTask, AcceptBid, CompleteTask
- MCP tools: post_task, bid_task, accept_bid, complete_task, list_tasks
- ExpiryWorker background goroutine for deadline-based task cancellation
- Channel type enforcement (auction ops only on auction channels)
- Agent cannot bid on own task, only poster accepts bids, only assignee completes
- Wired into main.go with graceful shutdown
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
