Restore trim_conversation: dedup memories, evict to budget, snap boundary

Ported the old trim_entries logic to the new AstNode types:
- Phase 1: Dedup Memory nodes by key (keep last), drop DMN entries
- Phase 2: While over budget, evict lowest-scored memory (if memories
  > 50% of conv tokens) or oldest conversation entry
- Phase 3: Snap to User message boundary at start

Called from compact() which runs on startup and on /compact.

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

src/agent/mod.rs

@@ -555,6 +555,8 @@ impl Agent {
 
         self.load_startup_journal().await;
 
+        self.context.lock().await.trim_conversation();
+
         let mut st = self.state.lock().await;
         st.generation += 1;
         st.last_prompt_tokens = 0;