Kill TextDelta, Info — UiMessage is dead. RAII ActivityGuards replace all status feedback

Streaming text now goes directly to agent entries via append_streaming().
sync_from_agent diffs the growing entry each tick. The streaming entry
is popped when the response completes; build_response_message pushes
the final version.

All status feedback uses RAII ActivityGuards:
- push_activity() for long-running work (thinking, streaming, scoring)
- notify() for instant feedback (compacted, DMN state changes, commands)
- Guards auto-remove on Drop, appending "(complete)" and lingering 5s
- expire_activities() cleans up timed-out notifications on render tick

UiMessage enum reduced to a single Info variant with zero sends.
The channel infrastructure remains for now (Mind/Agent still take
UiSender in signatures) — mechanical cleanup for a follow-up.

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

src/mind/mod.rs

@@ -259,6 +259,7 @@ impl Mind {
                     let mut ag = self.agent.lock().await;
                     if ag.last_prompt_tokens() > threshold {
                         ag.compact();
+                        ag.notify("compacted");
                     }
                 }
                 MindCommand::Score => {
@@ -381,7 +382,7 @@ impl Mind {
                     let _ = self.turn_watch.send(false);
 
                     if let Some(name) = model_switch {
-                        crate::user::chat::cmd_switch_model(&self.agent, &name, &self.ui_tx).await;
+                        crate::user::chat::cmd_switch_model(&self.agent, &name).await;
                     }
 
                     // Post-turn maintenance