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/modes/rpc/rpc-mode.ts

@@ -160,6 +160,10 @@ export async function runRpcMode(session: AgentSession): Promise<never> {
 			// Component factories are not supported in RPC mode - would need TUI access
 		},
 
+		setFooter(_factory: unknown): void {
+			// Custom footer not supported in RPC mode - requires TUI access
+		},
+
 		setTitle(title: string): void {
 			// Fire and forget - host can implement terminal title control
 			output({