/instructions command does not show ~/.copilot/instructions/*.instructions.md user-level files

### Describe the bug

The `/instructions` command shows an empty picker ("No instructions") even when user-level instruction files exist at `~/.copilot/instructions/*.instructions.md`.

The CLI help text lists `\C:\Users\Eric/.copilot/copilot-instructions.md` as a respected location, and the v0.0.412 changelog confirms `~/.copilot/instructions/*.instructions.md` support was added — but **neither location appears in the `/instructions` picker**.

### Affected version

`GitHub Copilot CLI 1.0.22`

### Steps to reproduce the behavior

1. Create `~/.copilot/instructions/global.instructions.md` with any content (no frontmatter)
2. Launch `copilot` in any directory (tested both git repo and non-git directory)
3. Run `/instructions`
4. **Observed:** picker is empty — the file is not listed

Also reproduced with the legacy `~/.copilot/copilot-instructions.md` path — also not shown.

### Expected behavior

`/instructions` should list `~/.copilot/instructions/global.instructions.md` with an `[external]` label, as suggested by the v1.0.4 changelog entry:

> "Show individual instruction file names in /instructions picker with [external] labels for injected files"

The instructions **are** being injected into prompts (the CLI help text and docs confirm these paths are respected), but the files are invisible to the `/instructions` picker.

### Additional context

- **OS:** Windows 11
- **Terminal:** Windows Terminal
- **Shell:** PowerShell 7
- The user-level instructions **do** appear to be loaded (content is reflected in agent behavior), so this is a display/enumeration bug in the `/instructions` command specifically
- Possibly related: the issue may be worse or exclusive to non-git-repo working directories — `/instructions` may only enumerate files discovered via git-root traversal, inadvertently skipping the user-level `~/.copilot/instructions/` directory

Bắt đầu bằng cách tái hiện /instructions với các tệp cấp người dùng tại ~/.copilot/instructions/*.instructions.md và ~/.copilot/copilot-instructions.md trong cả thư mục git và không phải git. Theo dõi các đường dẫn phát hiện instruction và liệt kê của picker, sau đó xác minh rằng các tệp cấp người dùng được phát hiện xuất hiện với nhãn [external], trong khi việc chèn prompt hiện có vẫn hoạt động.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.