feat(worktree): add Git worktree support for isolated parallel sessions

Introduces a central Git worktree management system to enable isolated, parallel agent orchestration. This allows running multiple Gemini sessions on different branches within the same repository without file contention.

Key Features:
- Added 'WorktreeService' in @google/gemini-cli-core to manage creation, lifecycle, and cleanup of Git worktrees.
- Implemented smart cleanup logic that detects untracked files and new commits, automatically preserving worktrees with uncommitted work.
- Added '--worktree' (-w) flag to the CLI to launch sessions in fresh, isolated environments.
- Enhanced the session exit experience with detailed status messages and instructions for resuming work in preserved worktrees.

docs/cli/cli-reference.md

@@ -50,6 +50,7 @@ These commands are available within the interactive REPL.
 | `--model`                        | `-m`  | string  | `auto`    | Model to use. See [Model Selection](#model-selection) for available values.                                                                                            |
 | `--prompt`                       | `-p`  | string  | -         | Prompt text. Appended to stdin input if provided. Forces non-interactive mode.                                                                                         |
 | `--prompt-interactive`           | `-i`  | string  | -         | Execute prompt and continue in interactive mode                                                                                                                        |
+| `--worktree`                     | `-w`  | string  | -         | Start Gemini in a new git worktree. If no name is provided, one is generated automatically. Requires `experimental.worktrees: true` in settings.                       |
 | `--sandbox`                      | `-s`  | boolean | `false`   | Run in a sandboxed environment for safer execution                                                                                                                     |
 | `--approval-mode`                | -     | string  | `default` | Approval mode for tool execution. Choices: `default`, `auto_edit`, `yolo`                                                                                              |
 | `--yolo`                         | `-y`  | boolean | `false`   | **Deprecated.** Auto-approve all actions. Use `--approval-mode=yolo` instead.                                                                                          |