fix(nodes): resolve default node when multiple canvas-capable nodes are connected

`pickDefaultNode()` returned null when multiple connected canvas-capable
nodes existed and none matched the local Mac heuristic. This caused
"node required" errors for agents (especially sub-agents) calling the
canvas tool without an explicit node parameter.

In multi-node setups, any canvas-capable node is a valid target — the
receiving node broadcasts A2UI surfaces to all other connected devices.
Fall back to the first connected candidate instead of failing.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/agents/tools/nodes-utils.ts

@@ -45,7 +45,10 @@ function pickDefaultNode(nodes: NodeListNode[]): NodeListNode | null {
     return local[0];
   }
 
-  return null;
+  // Multiple candidates — pick the first connected canvas-capable node.
+  // For A2UI and other canvas operations, any node works since multi-node
+  // setups broadcast surfaces across devices.
+  return candidates[0] ?? null;
 }
 
 export async function listNodes(opts: GatewayCallOptions): Promise<NodeListNode[]> {