Function setup_merge_driver 

pub fn setup_merge_driver() -> Result<Vec<MergeDriverSetupStep>, String>

Configure git to use murk’s custom merge driver for .murk files.

1. Ensures .gitattributes contains *.murk merge=murk.
2. Runs git config merge.murk.name and git config merge.murk.driver.

Returns the steps that were performed.