`/add-dir` works with `~` but not `$HOME` for home directories

### Describe the bug

`$HOME` is a commonly used to refer to the home directory, and I referred to it when trying to add a directory to the allow list. However, Copilot CLI doesn't seem to know how to expand it the same way it does for `~`. As a result, the directory was not successfully added to the allow list _despite_ `/add-dir` saying it was "added" which was misleading.

https://github.com/user-attachments/assets/3cf5c9e0-82e1-4c62-b5de-6b2fa06fd7f8

### Affected version

0.0.330; Commit: a8bfefe

### Steps to reproduce the behavior

1. Run `/add-dir` with a path that refers to `$HOME` (or maybe any variable)
- You should see: `Added directory to allowed list: `
2. Run `/list-dirs`
- You should see: new path _not_ listed

### Expected behavior

A few possibilities here, but the gist is that I would've expected **any behavior that was clear about what worked / what didn't**. For example:

1. The new path to be added (in raw or expanded form)
1. A successful message _was not shown_ when the path wasn't actually added
1. Support for tab-completion so that the path can be auto-expanded, increasingly the likelihood of it being added successfully

### Additional context

- macOS on ARM
- iTerm2
- zsh