Copilot does not use output_path setting

[drewbitt]

[agents.copilot]
enabled = true
output_path = "AGENTS.md"

will still write to copilot-instructions.md

[intellectronica]

I think now that copilot in vscode supports AGENTS.md, it's time to remove the support for vendor-specific location. I'd like to stick to the standards as much as possible.