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/openai.rs

@@ -9,7 +9,7 @@ use reqwest::Client;
 use tokio::sync::mpsc;
 
 use super::types::*;
-use crate::user::ui_channel::{UiMessage, UiSender};
+use crate::user::ui_channel::UiSender;
 use super::StreamEvent;
 
 /// Stream SSE events from an OpenAI-compatible endpoint, sending
@@ -66,7 +66,6 @@ pub(super) async fn stream_events(
         &request,
         ("Authorization", &format!("Bearer {}", api_key)),
         &[],
-        ui_tx,
         &debug_label,
         request_json.as_deref(),
     )
@@ -105,7 +104,7 @@ pub(super) async fn stream_events(
         };
 
         if let Some(ref u) = chunk.usage {
-            let _ = tx.send(StreamEvent::Usage(u.clone();
+            let _ = tx.send(StreamEvent::Usage(u.clone()));
             usage = chunk.usage;
         }
 
@@ -126,7 +125,7 @@ pub(super) async fn stream_events(
                 reasoning_chars += r.len();
                 has_reasoning = true;
                 if !r.is_empty() {
-                    let _ = tx.send(StreamEvent::Reasoning(r.clone();
+                    let _ = tx.send(StreamEvent::Reasoning(r.clone()));
                 }
             }
             if let Some(ref r) = choice.delta.reasoning_details {
@@ -143,7 +142,7 @@ pub(super) async fn stream_events(
                     first_content_at = Some(reader.stream_start.elapsed());
                 }
                 content_len += text_delta.len();
-                let _ = tx.send(StreamEvent::Content(text_delta.clone();
+                let _ = tx.send(StreamEvent::Content(text_delta.clone()));
             }
 
             if let Some(ref tc_deltas) = choice.delta.tool_calls {