Remove dead code: append_text, needs_assistant_marker, target param

append_text was the TextDelta streaming handler — replaced by
append_streaming on Agent entries. needs_assistant_marker tracked
turn boundaries for the old message path. target removed from
Agent::turn — routing now determined by entry content.

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

src/mind/mod.rs

@@ -310,7 +310,7 @@ impl Mind {
                     let agent = self.agent.clone();
                     let result_tx = self.turn_tx.clone();
                     self.shared.lock().unwrap().turn_handle = Some(tokio::spawn(async move {
-                        let result = Agent::turn(agent, &input, target).await;
+                        let result = Agent::turn(agent, &input).await;
                         let _ = result_tx.send((result, target)).await;
                     }));
                 }