Add a WebSocket endpoint at /ws/nodes where external processes and
devices can connect and advertise their capabilities at runtime.
The gateway tracks connected nodes in a NodeRegistry and exposes
their capabilities as dynamically available tools via NodeTool.
- Add src/gateway/nodes.rs: WebSocket endpoint, NodeRegistry, protocol
- Add src/tools/node_tool.rs: Tool trait wrapper for node capabilities
- Add NodesConfig to config schema (disabled by default)
- Wire /ws/nodes route into gateway router
- Add NodeRegistry to AppState and all test constructions
- Re-export NodesConfig and NodeTool from module roots
Closes#3093
