feat(sync): replace WS+DO transport with unified HTTP sync

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)

internal/config/config_test.go

@@ -21,8 +21,8 @@ func TestDefault(t *testing.T) {
 	if cfg.General.Country != "US" {
 		t.Errorf("default Country = %q, want US", cfg.General.Country)
 	}
-	if cfg.Daemon.HeartbeatInterval != "30s" {
-		t.Errorf("default HeartbeatInterval = %q, want 30s", cfg.Daemon.HeartbeatInterval)
+	if cfg.Daemon.StatusInterval != "" {
+		t.Errorf("default StatusInterval = %q, want empty", cfg.Daemon.StatusInterval)
 	}
 }