/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

Inizia riproducendo /instructions con file a livello utente in ~/.copilot/instructions/*.instructions.md e ~/.copilot/copilot-instructions.md, sia nelle directory git sia in quelle non-git. Traccia i percorsi di individuazione delle istruzioni e di enumerazione del picker, quindi verifica che i file a livello utente individuati compaiano con etichette [external], mentre la prompt injection esistente continui a funzionare.

Scritto dal modello di indicizzazione a partire dal testo della issue.