`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>
