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_

首先追蹤現有的 /cd 命令、其狀態列更新，以及工具呼叫如何使用目前的目錄；issue 未指定特定檔案或測試。當 LLM 可以切換到一個 worktree，讓狀態列顯示新的路徑、分支或 PR，且後續工具呼叫在該處執行時，即表示完成。

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