Skills beyond alphabetical position ~32 appear unreachable when many skills are installed

### Describe the bug

When ~63 skills are installed in `~/.copilot/skills/`, the system prompt visible to the model contains:

```

```

In testing, a custom skill that sorts alphabetically around position 36 was never selected by the model.

After manually reducing installed skills to 32, the same custom skill was selected correctly in subsequent sessions.

I can't confirm the exact internal mechanism, but the observed behavior suggests that skills listed after the cutoff point may not be reachable by the model.

### Affected version

0.0.409

### Steps to reproduce the behavior

1. Install 60+ skills in `~/.copilot/skills/`
2. Start a new Copilot CLI session
3. The system prompt shows ``
4. Ask the agent to invoke a skill whose name sorts alphabetically after position ~32
5. In my testing, the model selected a different, earlier-sorting skill instead

### Expected behavior

- Skills should have an equal chance of being selected regardless of their name
- If a limit exists, the user should be informed which skills are active and which are not
- Suggestions: relevance-based ordering, user-configurable skill priority, or a way to pin important skills

### Additional context

- OS: Windows 11, Windows Terminal, PowerShell
- Workaround: manually curate to ≤32 skills total
- Users who install skills from collections (e.g., awesome-copilot-skills) can easily exceed 32 without realizing some become unreachable