Add LLM invocable change directory tool

### Describe the feature or problem you'd like to solve

Keep the pwd up to date in the status bar when copilot switches to another worktree

### Proposed solution

Currently the user can run `/cd` to move directories and the status line gets updated and all tool invocations are then run in that directory.

The LLM doesn't have a corresponding tool. Often I'll tell it to go checkout a specific branch and PR into a new/existing worktree and it will do it but then I have to stop it and get the path and manually run /cd to that path so visually I can see the directory/branch name/PR in the status line.

### Example prompts or workflows

Checkout branch X into a worktree and then cd into that worktree and begin addressing the feedback.

### Additional context

_No response_

Start by tracing the existing /cd command, its status-bar update, and how tool invocations use the current directory; the issue names no specific files or tests. Done means the LLM can change into a worktree so the status line shows the new path, branch, or PR and later tool invocations run there.

Written by the indexing model from the issue text.