Memory scores on entries, not a separate Vec

ConversationEntry::Memory gains score: Option<f64>. The scorer
writes scores directly onto entries when results arrive. Removes
Agent.memory_scores Vec and the memory_scores parameter from
context_state_summary().

Scores are serialized to/from the conversation log as memory_score.

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

src/mind/dmn.rs

@@ -428,7 +428,7 @@ impl Subconscious {
                                 ));
                                 msg.stamp();
                                 ag.push_entry(ConversationEntry::Memory {
-                                    key: key.to_string(), message: msg,
+                                    key: key.to_string(), message: msg, score: None,
                                 });
                             }
                         }