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/agent/client.go

@@ -40,27 +40,6 @@ func (c *Client) Register(ctx context.Context, req RegisterRequest) (*RegisterRe
 	return &resp, nil
 }
 
-// Heartbeat sends a periodic keep-alive signal and returns server directives.
-func (c *Client) Heartbeat(ctx context.Context, req HeartbeatRequest) (*HeartbeatResponse, error) {
-	var resp HeartbeatResponse
-	if err := c.doPost(ctx, "/api/internal/agent/heartbeat", req, &resp); err != nil {
-		return nil, fmt.Errorf("heartbeat: %w", err)
-	}
-	return &resp, nil
-}
-
-// ClaimTasks polls for pending download tasks and claims them atomically.
-// Also returns any stream requests for completed downloads.
-func (c *Client) ClaimTasks(ctx context.Context, agentID string) (*TasksResponse, error) {
-	url := fmt.Sprintf("/api/internal/agent/tasks?agentId=%s", agentID)
-	var resp TasksResponse
-	if err := c.doGet(ctx, url, &resp); err != nil {
-		return nil, fmt.Errorf("claim tasks: %w", err)
-	}
-	return &resp, nil
-}
-
-// ReportStatus reports download progress or completion for a task.
 // Deregister notifies the server that the agent is shutting down.
 func (c *Client) Deregister(ctx context.Context, agentID string) error {
 	req := struct {
@@ -91,6 +70,16 @@ func (c *Client) BatchReportStatus(ctx context.Context, updates []StatusUpdate)
 	return &resp, nil
 }
 
+// Sync sends the CLI's full state and receives all pending server actions.
+// This is the single endpoint for bidirectional state synchronization.
+func (c *Client) Sync(ctx context.Context, req SyncRequest) (*SyncResponse, error) {
+	var resp SyncResponse
+	if err := c.doPost(ctx, "/api/internal/agent/sync", req, &resp); err != nil {
+		return nil, fmt.Errorf("sync: %w", err)
+	}
+	return &resp, nil
+}
+
 // ---------------------------------------------------------------------------
 // Usenet endpoints
 // ---------------------------------------------------------------------------