Replace the WebSocket + Cloudflare Durable Object architecture with a
single POST /sync endpoint. The CLI now operates autonomously with local
state (tasks.json) and syncs bidirectionally via adaptive-interval HTTP
polling (3s watching, 60s idle).
- Remove transport_ws, transport_hybrid, transport_http (~2,600 lines)
- Add SyncClient with adaptive interval loop
- Add LocalState for CLI-side task persistence
- Add TaskStateFromUpdate() helper (DRY)
- Extract finalize() to deduplicate processTask/processTaskRetry
- Consolidate shortID() into agent.ShortID (was in 3 packages)
- Wire GetActiveCount so `unarr status` shows active tasks
- Remove poll_interval, heartbeat_interval, ws_url from config
- Simplify ProgressReporter (sync replaces direct HTTP reporting)
