Function merge_groups 

pub fn merge_groups(
    existing: &[SemanticGroup],
    new_assignments: &[SemanticGroup],
    delta: &DiffDelta,
) -> Vec<SemanticGroup>

Merge new LLM grouping assignments into existing groups.

Steps:

1. Clone existing groups.
2. Remove entries for removed_files and modified_files (stale data).
3. For each group in new_assignments:
   • If label matches an existing group (case-insensitive), merge changes into it.
   • Otherwise, append as a new group.
4. Remove empty groups.