Add ctx.ui.setFooter() for extensions to replace footer component

Extensions can now replace the built-in footer with a custom component:
- setFooter(factory) replaces with custom component
- setFooter(undefined) restores built-in footer

Includes example extension demonstrating context usage display.

Closes #481

packages/coding-agent/src/core/extensions/loader.ts

@@ -88,6 +88,7 @@ function createNoOpUIContext(): ExtensionUIContext {
 		notify: () => {},
 		setStatus: () => {},
 		setWidget: () => {},
+		setFooter: () => {},
 		setTitle: () => {},
 		custom: async () => undefined as never,
 		setEditorText: () => {},