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/StartScreen.tsx

@@ -125,6 +125,24 @@ export function StartScreen({
       onDeleteSession(sessions[selectedIndex]);
       return;
     }
+
+    // 'n' to create new session
+    if (input === 'n') {
+      onCreateSession();
+      return;
+    }
+
+    // 'r' to refresh session list
+    if (input === 'r') {
+      onRefresh();
+      return;
+    }
+
+    // 'q' to quit
+    if (input === 'q') {
+      onExit();
+      return;
+    }
   });
 
   return (