Function merge_codegraph_edges 

pub fn merge_codegraph_edges(
    kg: &mut KnowledgeGraph,
    project_root: &Path,
) -> Result<usize>

Merge edges from a CodeGraph SQLite database into an existing KnowledgeGraph.

Looks for .codegraph/codegraph.db under project_root. If not found, or if no matching nodes exist, returns Ok(0).

Returns the number of edges merged.