excludeAgent does not work on subagents

### Describe the bug

As of right now, excludeAgent in the frontmatter block does not work on subagents.

This limits custom instructions which are meant for main agent but not subagents.

### Affected version

GitHub Copilot CLI 1.0.10

### Steps to reproduce the behavior

1. Set up custom instructions with excludeAgent
2. Test if subagents are following custom instructions

### Expected behavior

excludeAgent in the frontmatter block would extend to subagents.

### Additional context

_No response_