feat(cli): Add ShellModeIndicator component

This commit introduces a new ShellModeIndicator component to visually signify when shell mode is active.

- Displays "shell mode enabled (! to toggle)" in the UI.
- The AutoAcceptIndicator is now hidden when shell mode is active to prevent UI clutter.

packages/cli/src/ui/components/ShellModeIndicator.tsx

@@ -0,0 +1,18 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import React from 'react';
+import { Box, Text } from 'ink';
+import { Colors } from '../colors.js';
+
+export const ShellModeIndicator: React.FC = () => (
+  <Box>
+    <Text color={Colors.AccentYellow}>
+      shell mode enabled
+      <Text color={Colors.SubtleComment}> (! to toggle)</Text>
+    </Text>
+  </Box>
+);