rename: sort candidates by least-recently visited

Instead of a hard 7-day cutoff, sort rename candidates so the
least-recently visited come first. Naturally prioritizes unseen
nodes while allowing revisits once everything's been through.

poc-memory/src/agents/prompts.rs

@@ -275,7 +275,8 @@ pub fn format_rename_candidates(store: &Store, count: usize) -> (Vec<String>, St
         .map(|(k, n)| (k.as_str(), n))
         .collect();
 
-    candidates.sort_by(|a, b| b.1.timestamp.cmp(&a.1.timestamp));
+    // Least-recently visited first — naturally prioritizes unseen nodes
+    candidates.sort_by_key(|(key, _)| store.last_visited(key, "rename"));
     candidates.truncate(count);
 
     let keys: Vec<String> = candidates.iter().map(|(k, _)| k.to_string()).collect();