refactor(tui): clean up unused code and consolidate input handling

- Remove unused imports (Text, execSync, writeFileSync, React)
- Move n/r/q key handlers from App.tsx to StartScreen component
- Mark unused onClose prop in HelpOverlay with underscore prefix
- Consolidate keyboard handling in components that display the controls

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/tui/index.tsx

@@ -24,7 +24,6 @@
  * @see {@link ./hooks/useSessionManager.ts} for state management
  */
 
-import React from 'react';
 import { render } from 'ink';
 import { App } from './App.js';