Function seed_sandbox_manifests 

pub fn seed_sandbox_manifests(
    working_dir: &Path,
    sandbox_dir: &Path,
    plugins: &[&str],
) -> Result<()>

Seed a sandbox with plugin-identified project manifests (Cargo.toml, pyproject.toml, etc.) so that build/test commands can find them.

Walks the workspace looking for each plugin’s key_files() and copies any that exist into the sandbox at the same relative path.