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.