cleanup: remove dead placeholder code, use RPC for identity loading

- links() in memory.rs: use cached_store() instead of MemoryNode::load()
- identity.rs: use memory_rpc for Store context loading
- defs.rs: delete dead placeholders (topology, nodes/episodes, health, split)
  - agents now use {{tool: graph_topology}} etc instead
- prompts.rs: delete unused format_split_plan_node()

Co-Authored-By: Kent Overstreet <kent.overstreet@linux.dev>

src/agent/tools/memory.rs

@@ -301,7 +301,9 @@ async fn search(args: &serde_json::Value) -> Result<String> {
 
 async fn links(args: &serde_json::Value) -> Result<String> {
     let key = get_str(args, "key")?;
-    let node = MemoryNode::load(key)
+    let arc = cached_store().await?;
+    let store = arc.lock().await;
+    let node = MemoryNode::from_store(&store, key)
         .ok_or_else(|| anyhow::anyhow!("node not found: {}", key))?;
     let mut out = format!("Neighbors of '{}':\n", key);
     for (target, strength, is_new) in &node.links {