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/hooks/useSessionManager.ts

@@ -24,7 +24,7 @@
  */
 
 import { useState, useEffect, useCallback, useRef } from 'react';
-import { existsSync, readFileSync, writeFileSync, watchFile, unwatchFile, unlinkSync } from 'fs';
+import { existsSync, readFileSync, watchFile, unwatchFile, unlinkSync } from 'fs';
 import { homedir } from 'os';
 import { join } from 'path';
 import { execSync } from 'child_process';