Use ActivityGuard for context overflow retry progress

Instead of two separate notifications piling up on the status bar,
use a single ActivityGuard that updates in place during overflow
retries and auto-completes when the turn finishes.

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

src/agent/mod.rs

@@ -318,6 +318,7 @@ impl Agent {
         }
 
         let mut overflow_retries: u32 = 0;
+        let mut overflow_activity: Option<ActivityGuard> = None;
         let mut empty_retries: u32 = 0;
         let mut ds = DispatchState::new();
 
@@ -382,8 +383,12 @@ impl Agent {
                         }
                         if overflow_retries < 2 {
                             overflow_retries += 1;
-                            agent.state.lock().await.notify(
-                                format!("context overflow — retrying ({}/2)", overflow_retries));
+                            let msg = format!("context overflow — compacting ({}/2)", overflow_retries);
+                            match &overflow_activity {
+                                Some(a) => a.update(&msg).await,
+                                None => overflow_activity = Some(
+                                    start_activity(&agent, &msg).await),
+                            }
                             agent.compact().await;
                             continue;
                         }