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/test/compaction-extensions.test.ts

@@ -122,6 +122,7 @@ describe.skipIf(!API_KEY)("Compaction extensions", () => {
 				notify: () => {},
 				setStatus: () => {},
 				setWidget: () => {},
+				setFooter: () => {},
 				setTitle: () => {},
 				custom: async () => undefined as never,
 				setEditorText: () => {},