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/agent/api/mod.rs

@@ -20,7 +20,7 @@ use tokio::sync::mpsc;
 
 use crate::agent::tools::{self as agent_tools, summarize_args, ActiveToolCall};
 pub use types::ToolCall;
-use crate::user::ui_channel::{UiMessage, UiSender, StreamTarget};
+use crate::user::ui_channel::UiSender;
 
 /// A JoinHandle that aborts its task when dropped.
 pub struct AbortOnDrop(tokio::task::JoinHandle<()>);
@@ -130,7 +130,7 @@ impl ApiClient {
                 &reasoning_effort, sampling, priority,
             ).await;
             if let Err(e) = result {
-                let _ = tx.send(StreamEvent::Error(e.to_string();
+                let _ = tx.send(StreamEvent::Error(e.to_string()));
             }
         });
 
@@ -207,7 +207,6 @@ pub(crate) async fn send_and_check(
     body: &impl serde::Serialize,
     auth_header: (&str, &str),
     extra_headers: &[(&str, &str)],
-    ui_tx: &UiSender,
     debug_label: &str,
     request_json: Option<&str>,
 ) -> Result<reqwest::Response> {
@@ -619,8 +618,6 @@ pub struct StreamResult {
 /// - UI forwarding (text deltas, reasoning, tool call notifications)
 pub async fn collect_stream(
     rx: &mut mpsc::UnboundedReceiver<StreamEvent>,
-    ui_tx: &UiSender,
-    target: StreamTarget,
     agent: &std::sync::Arc<tokio::sync::Mutex<super::Agent>>,
     active_tools: &crate::user::ui_channel::SharedActiveTools,
 ) -> StreamResult {
@@ -633,12 +630,13 @@ pub async fn collect_stream(
     let mut error = None;
     let mut first_content = true;
     let mut display_buf = String::new();
+    let mut _streaming_guard: Option<super::ActivityGuard> = None;
 
     while let Some(event) = rx.recv().await {
         match event {
             StreamEvent::Content(text) => {
                 if first_content {
-                    if let Ok(mut ag) = agent.try_lock() { ag.activity = "streaming...".into(); }
+                    _streaming_guard = Some(super::start_activity(agent, "streaming...").await);
                     first_content = false;
                 }
                 content.push_str(&text);
@@ -683,7 +681,7 @@ pub async fn collect_stream(
                     if let Some(pos) = display_buf.find("<tool_call>") {
                         let before = &display_buf[..pos];
                         if !before.is_empty() {
-                            let _ = ui_tx.send(UiMessage::TextDelta(before.to_string(), target));
+                            if let Ok(mut ag) = agent.try_lock() { ag.append_streaming(before); }
                         }
                         display_buf.clear();
                         in_tool_call = true;
@@ -693,7 +691,7 @@ pub async fn collect_stream(
                         if safe > 0 {
                             let flush = display_buf[..safe].to_string();
                             display_buf = display_buf[safe..].to_string();
-                            let _ = ui_tx.send(UiMessage::TextDelta(flush, target));
+                            if let Ok(mut ag) = agent.try_lock() { ag.append_streaming(&flush); }
                         }
                     }
                 }