unify memory tracking: entries are the single source of truth

Memory tool results (memory_render) are now pushed as
ConversationEntry::Memory with the node key, instead of plain
Messages. Remove loaded_nodes from ContextState — the debug
screen reads memory info from Memory entries in the conversation.

Surfaced memories from surface-observe are pushed as separate
Memory entries, reflections as separate system-reminder messages.
User input is no longer polluted with hook output.

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

src/subconscious/subconscious.rs

@@ -13,6 +13,7 @@ use std::time::{Duration, Instant, SystemTime};
 pub use crate::session::HookSession;
 
 /// Output from a single agent orchestration cycle.
+#[derive(Default)]
 pub struct AgentCycleOutput {
     /// Memory node keys surfaced by surface-observe.
     pub surfaced_keys: Vec<String>,