feat(cli): Introduce toggleable shell mode with enhanced UI

- Implements a toggleable shell mode, removing the need to prefix every command with `!`.
- Users can now enter and exit shell mode by typing `!` as the first character in an empty input prompt.
- The input prompt visually indicates active shell mode with a distinct color and `! ` prefix.
- Shell command history items (`user_shell`) are now visually differentiated from regular user messages.
- This provides a cleaner and more streamlined user experience for frequent shell interactions.

Fixes https://b.corp.google.com/issues/418509745

packages/cli/src/ui/components/messages/UserShellMessage.tsx

@@ -0,0 +1,25 @@
+/**
+ * @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';
+
+interface UserShellMessageProps {
+  text: string;
+}
+
+export const UserShellMessage: React.FC<UserShellMessageProps> = ({ text }) => {
+  // Remove leading '!' if present, as App.tsx adds it for the processor.
+  const commandToDisplay = text.startsWith('!') ? text.substring(1) : text;
+
+  return (
+    <Box>
+      <Text color={Colors.AccentCyan}>$ </Text>
+      <Text>{commandToDisplay}</Text>
+    </Box>
+  );
+};