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/HistoryItemDisplay.tsx

@@ -7,6 +7,7 @@
 import React from 'react';
 import type { HistoryItem } from '../types.js';
 import { UserMessage } from './messages/UserMessage.js';
+import { UserShellMessage } from './messages/UserShellMessage.js';
 import { GeminiMessage } from './messages/GeminiMessage.js';
 import { InfoMessage } from './messages/InfoMessage.js';
 import { ErrorMessage } from './messages/ErrorMessage.js';
@@ -28,6 +29,7 @@ export const HistoryItemDisplay: React.FC<HistoryItemDisplayProps> = ({
   <Box flexDirection="column" key={item.id}>
     {/* Render standard message types */}
     {item.type === 'user' && <UserMessage text={item.text} />}
+    {item.type === 'user_shell' && <UserShellMessage text={item.text} />}
     {item.type === 'gemini' && (
       <GeminiMessage
         text={item.text}