feat(hooks): add setWidgetComponent for custom TUI components

- New ctx.ui.setWidgetComponent(key, factory) method
- Allows custom Component to render as widget without taking focus
- Unlike custom(), widget components render inline above editor
- Components are disposed when cleared or replaced
- Falls back to no-op in RPC/print modes

packages/coding-agent/src/modes/rpc/rpc-mode.ts

@@ -142,6 +142,10 @@ export async function runRpcMode(session: AgentSession): Promise<never> {
 			} as RpcHookUIRequest);
 		},
 
+		setWidgetComponent(): void {
+			// Custom components not supported in RPC mode - host would need to implement
+		},
+
 		async custom() {
 			// Custom UI not supported in RPC mode
 			return undefined as never;