feat(worktree): add Git worktree support for isolated parallel sessions (#22973)

docs/cli/git-worktrees.md

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+# Git Worktrees (experimental)
+
+When working on multiple tasks at once, you can use Git worktrees to give each
+Gemini session its own copy of the codebase. Git worktrees create separate
+working directories that each have their own files and branch while sharing the
+same repository history. This prevents changes in one session from colliding
+with another.
+
+Learn more about [session management](./session-management.md).
+
+<!-- prettier-ignore -->
+> [!NOTE]
+> This is an experimental feature currently under active development. Your
+> feedback is invaluable as we refine this feature. If you have ideas,
+> suggestions, or encounter issues:
+>
+> - [Open an issue](https://github.com/google-gemini/gemini-cli/issues/new?template=bug_report.yml) on GitHub.
+> - Use the **/bug** command within Gemini CLI to file an issue.
+
+Learn more in the official Git worktree
+[documentation](https://git-scm.com/docs/git-worktree).
+
+## How to enable Git worktrees
+
+Git worktrees are an experimental feature. You must enable them in your settings
+using the `/settings` command or by manually editing your `settings.json` file.
+
+1.  Use the `/settings` command.
+2.  Search for and set **Enable Git Worktrees** to `true`.
+
+Alternatively, add the following to your `settings.json`:
+
+```json
+{
+  "experimental": {
+    "worktrees": true
+  }
+}
+```
+
+## How to use Git worktrees
+
+Use the `--worktree` (`-w`) flag to create an isolated worktree and start Gemini
+CLI in it.
+
+- **Start with a specific name:** The value you pass becomes both the directory
+  name (within `.gemini/worktrees/`) and the branch name.
+
+  ```bash
+  gemini --worktree feature-search
+  ```
+
+- **Start with a random name:** If you omit the name, Gemini generates a random
+  one automatically (for example, `worktree-a1b2c3d4`).
+
+  ```bash
+  gemini --worktree
+  ```
+
+<!-- prettier-ignore -->
+> [!NOTE]
+> Remember to initialize your development environment in each new
+> worktree according to your project's setup. Depending on your stack, this
+> might include running dependency installation (`npm install`, `yarn`), setting
+> up virtual environments, or following your project's standard build process.
+
+## How to exit a Git worktree session
+
+When you exit a worktree session (using `/quit` or `Ctrl+C`), Gemini leaves the
+worktree intact so your work is not lost. This includes your uncommitted changes
+(modified files, staged changes, or untracked files) and any new commits you
+have made.
+
+Gemini prioritizes a fast and safe exit: it **does not automatically delete**
+your worktree or branch. You are responsible for cleaning up your worktrees
+manually once you are finished with them.
+
+When you exit, Gemini displays instructions on how to resume your work or how to
+manually remove the worktree if you no longer need it.
+
+## Resuming work in a Git worktree
+
+To resume a session in a worktree, navigate to the worktree directory and start
+Gemini CLI with the `--resume` flag and the session ID:
+
+```bash
+cd .gemini/worktrees/feature-search
+gemini --resume <session_id>
+```
+
+## Managing Git worktrees manually
+
+For more control over worktree location and branch configuration, or to clean up
+a preserved worktree, you can use Git directly:
+
+- **Clean up a preserved Git worktree:**
+  ```bash
+  git worktree remove .gemini/worktrees/feature-search --force
+  git branch -D worktree-feature-search
+  ```
+- **Create a Git worktree manually:**
+  ```bash
+  git worktree add ../project-feature-search -b feature-search
+  cd ../project-feature-search && gemini
+  ```
+
+[Open an issue]: https://github.com/google-gemini/gemini-cli/issues