get_group_content: use RPC, delete store-based version

One function that uses memory_rpc (which handles daemon vs local).
Removes 65 lines of duplicate logic.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/agent/tools/memory.rs

@@ -573,4 +573,3 @@ async fn graph_health() -> Result<String> {
     let graph = store.build_graph();
     Ok(crate::subconscious::prompts::format_health_section(&store, &graph))
 }
-