Function get_subgraph 

pub fn get_subgraph<S: GraphStore + ?Sized>(
    store: &S,
    branch: &str,
    query: &str,
    depth: u8,
    direction: &str,
    options: AgentQueryOptions,
) -> Result<AgentSubgraphResponse>

Return a compact, exact-ID neighborhood digest. Only direct relationships are serialized as evidence; deeper traversal contributes coverage counts.