Function store_blob_at_ref 

pub fn store_blob_at_ref(
    repo_path: &Path,
    ref_name: &str,
    rel_path: &str,
    body: &[u8],
    message: &str,
) -> Result<ObjectId>

Store arbitrary blob content under a specific ref/path without touching the working tree.

This powers opensession share --git, which needs explicit ref/path control.