Function upsert_ssh_config_block 

pub fn upsert_ssh_config_block(
    existing: &str,
    name: &str,
    block: &str,
) -> String

Insert or replace the Host git-ark-<name> block within existing (the current contents of ~/.ssh/config), keyed by its Host line. Every other block is preserved untouched; a prior block for this name is replaced in place (idempotent re-runs of host add), otherwise the new block is appended.