split agent: filter candidates to semantic nodes only

Episodic nodes (journal entries, digests) are narratives that should
not be split even when large. Only semantic reference nodes that have
grown to cover multiple topics are candidates.

poc-memory/src/agents/prompts.rs

@@ -318,6 +318,7 @@ pub fn split_candidates(store: &Store) -> Vec<String> {
             !key.starts_with('_')
                 && !node.deleted
                 && node.content.len() > 2000
+                && matches!(node.node_type, crate::store::NodeType::Semantic)
         })
         .map(|(k, n)| (k.as_str(), n.content.len()))
         .collect();