Kill StatusUpdate, Activity, DmnAnnotation, ContextInfoUpdate, AgentUpdate

Status bar reads directly from Agent and MindState on each render tick.
Activity is now a field on Agent — set by agent code directly, read by
UI via try_lock. DmnAnnotation, ContextInfoUpdate, AgentUpdate were
already dead (no senders).

UiMessage down to 4 variants: TextDelta, Reasoning, Debug, Info.

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

src/agent/api/mod.rs

@@ -638,7 +638,7 @@ pub async fn collect_stream(
         match event {
             StreamEvent::Content(text) => {
                 if first_content {
-                    let _ = ui_tx.send(UiMessage::Activity("streaming...".into()));
+                    if let Ok(mut ag) = agent.try_lock() { ag.activity = "streaming...".into(); }
                     first_content = false;
                 }
                 content.push_str(&text);