feat(worktree): add /worktree slash command for interactive switching

Implements the /worktree slash command, allowing users to switch the
active session to a new isolated Git worktree without restarting the CLI.

Key changes:
- Added '/worktree' slash command with streamlined messaging.
- Updated 'Config' class to support runtime updates to the target directory.
- Introduced 'WorkingDirectoryChanged' event to notify UI and hooks of path changes.
- Added comprehensive tests and updated documentation.

docs/cli/git-worktrees.md

@@ -38,6 +38,10 @@ Alternatively, add the following to your `settings.json`:
 
 ## How to use Git worktrees
 
+There are two ways to use Git worktrees: at startup or mid-session.
+
+### Using the startup flag
+
 Use the `--worktree` (`-w`) flag to create an isolated worktree and start Gemini
 CLI in it.
 
@@ -55,6 +59,33 @@ CLI in it.
   gemini --worktree
   ```
 
+### Using the slash command
+
+If you are already in a Gemini session, you can switch to a new isolated
+worktree without restarting using the `/worktree` command.
+
+- **Switch with a specific name:**
+
+  ```text
+  /worktree feature-auth
+  ```
+
+- **Switch with a random name:**
+  ```text
+  /worktree
+  ```
+
+When you use `/worktree`, Gemini:
+
+1.  **Checks your current environment:** If you are already in a worktree, it
+    automatically cleans it up if there are no changes, or preserves it if there
+    are.
+2.  **Creates the new worktree:** Sets up a fresh copy of the codebase and a new
+    branch.
+3.  **Pivots the session:** Switches its working directory and reloads the
+    project context (memory and tools) so you can immediately start working in
+    isolation.
+
 > **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