Function load_guest_primary 

pub fn load_guest_primary(
    bin_path: &Path,
) -> Result<Option<GuestPrimarySessionV1>>

Load guest-primary metadata: workspace env path, then cwd .cargo-devshell/session.json, then legacy beside bin_path.

Returns Ok(None) if no file exists or format is unrecognized.

Errors

I/O errors, or invalid JSON (wrapped as InvalidData).