feat(cli): support Ctrl-Z suspension

- Implement Ctrl+Z suspension with robust terminal state handling.
- Rebind Undo/Redo to Alt+Z / Alt+Shift+Z to avoid conflicts.
- Fix macOS Option key character mapping for Undo/Redo.
- Improve type safety for app.rerender() in AppContainer.
- Add comprehensive suspension test suite.
- Fix environment leakage in core tests.
- Update keyboard shortcut documentation.

Fixes google-gemini#5018

Author: brtkwr

docs/cli/keyboard-shortcuts.md

@@ -119,7 +119,7 @@ available combinations.
 | Move focus from the shell back to Gemini.                                                             | `Shift + Tab`              |
 | Clear the terminal screen and redraw the UI.                                                          | `Ctrl + L`                 |
 | Restart the application.                                                                              | `R`                        |
-| Suspend the application (not yet implemented).                                                        | `Ctrl + Z`                 |
+| Suspend the CLI and move it to the background.                                                        | `Ctrl + Z`                 |
 
 <!-- KEYBINDINGS-AUTOGEN:END -->
 

packages/cli/src/config/keyBindings.ts

@@ -518,5 +518,5 @@ export const commandDescriptions: Readonly<Record<Command, string>> = {
   [Command.UNFOCUS_SHELL_INPUT]: 'Move focus from the shell back to Gemini.',
   [Command.CLEAR_SCREEN]: 'Clear the terminal screen and redraw the UI.',
   [Command.RESTART_APP]: 'Restart the application.',
-  [Command.SUSPEND_APP]: 'Suspend the application (not yet implemented).',
+  [Command.SUSPEND_APP]: 'Suspend the CLI and move it to the background.',
 };