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/user/mod.rs

@@ -345,7 +345,6 @@ pub async fn run(
 ) -> Result<()> {
     let agent = &mind.agent;
     let shared_mind = &mind.shared;
-    let turn_watch = mind.turn_watch();
     // UI-owned state
     let mut idle_state = crate::thalamus::idle::State::new();
     idle_state.load();