Copilot AH should not access files in non-trusted folders

When an Copilot SDK session's working directory is not inside a Git repository, customization discovery goes up the parent chain until the user's home directory to look for customization files.
As a result, project-scoped customizations can be inherited from every ancestor between the working directory and $HOME, including $HOME itself.

That's useful for mono-repo use cases, but Copilot should check whether these folders are trusted.

I filed the following issues against the SDK:
https://github.com/github/copilot-agent-runtime/issues/13531
Also it seems that it even goes into sibling directories
https://github.com/github/copilot-agent-runtime/issues/16814