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: () => {},

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

@@ -63,6 +63,7 @@ const noOpUIContext: ExtensionUIContext = {
 	notify: () => {},
 	setStatus: () => {},
 	setWidget: () => {},
+	setFooter: () => {},
 	setTitle: () => {},
 	custom: async () => undefined as never,
 	setEditorText: () => {},

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

@@ -65,6 +65,9 @@ export interface ExtensionUIContext {
 	setWidget(key: string, content: string[] | undefined): void;
 	setWidget(key: string, content: ((tui: TUI, theme: Theme) => Component & { dispose?(): void }) | undefined): void;
 
+	/** Set a custom footer component, or undefined to restore the built-in footer. */
+	setFooter(factory: ((tui: TUI, theme: Theme) => Component & { dispose?(): void }) | undefined): void;
+
 	/** Set the terminal window/tab title. */
 	setTitle(title: string): void;