Startup hint "No copilot instructions found" is misleading when user-level instructions exist

## Summary
Startup hint "No copilot instructions found" is misleading when user-level instructions exist at `~/.copilot/copilot-instructions.md`

## Environment
- **Product/Service**: GitHub Copilot CLI
- **Version**: v0.0.413-0
- **OS**: Windows 11

## Reproduction Steps
1. Create user-level instructions at `~/.copilot/copilot-instructions.md`
2. Launch `copilot` from a directory that is **not** inside a git repository (e.g., `$HOME` — which is typically not git-tracked but is where user-level instructions live)
3. Observe startup output

## Expected Behavior
The startup hint should either:
- Not appear when user-level instructions (`~/.copilot/copilot-instructions.md`) are detected, OR
- Distinguish between project-level and user-level instructions (e.g., "No **project** instructions found. User-level instructions loaded from ~/.copilot/copilot-instructions.md")

## Actual Behavior
Displays:
```
💡 No copilot instructions found. Run /init to generate a copilot-instructions.md file for this project.
```

This message appears even though user-level instructions at `~/.copilot/copilot-instructions.md` **are** being loaded and applied correctly. The hint creates unnecessary confusion — users believe their instructions are not working.

## Impact
**Low** — Cosmetic/UX issue. Instructions load correctly, but the message undermines developer confidence in their setup and may lead to unnecessary troubleshooting.

## Additional Context
- The CLI documentation confirms `$HOME/.copilot/copilot-instructions.md` is a supported instructions location
- The hint only checks for project-level instructions (`.github/copilot-instructions.md` in a git root) and ignores the user-level file
- Particularly confusing for users who primarily work from their home directory or non-repo folders
- The user home directory (`$HOME` / `%USERPROFILE%`) is typically not git-tracked, yet it is the standard location for user-level Copilot instructions — this makes the hint appear in the most common non-repo launch context