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/hooks/shellCommandProcessor.ts

@@ -38,7 +38,10 @@ export const useShellCommandProcessor = (
       const commandToExecute = rawQuery.trim().slice(1).trimStart();
 
       const userMessageTimestamp = Date.now();
-      addItemToHistory({ type: 'user', text: rawQuery }, userMessageTimestamp);
+      addItemToHistory(
+        { type: 'user_shell', text: rawQuery },
+        userMessageTimestamp,
+      );
 
       if (!commandToExecute) {
         addItemToHistory(