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/components/HelpOverlay.tsx

@@ -75,7 +75,7 @@ const SHORTCUT_GROUPS: ShortcutGroup[] = [
  * @param props.onClose - Callback invoked when overlay should close
  * @returns The help overlay element
  */
-export function HelpOverlay({ onClose }: HelpOverlayProps): React.ReactElement {
+export function HelpOverlay({ onClose: _onClose }: HelpOverlayProps): React.ReactElement {
   return (
     <Box flexDirection="column" padding={2}>
       {/* Header */}