Function rebase_with_fallbacks 

pub async fn rebase_with_fallbacks(
    repo_dir: &Path,
    base_branch: &str,
) -> RebaseOutcome

Rebase the current branch onto the latest origin/<base_branch> with fallbacks.

Tries rebase first. If that fails (merge conflicts), falls back to a merge commit. If both fail, returns MergeConflicts with the list of conflicting files – the working tree is left in a conflicted state so the caller can attempt agent-assisted resolution.