fix(agents): Keep a deleted task from being saved again

A save that ran after delete (from the turn ending or the agent exiting)
wrote the task file back, so the task returned on the next load. Deleting
now marks the task under the lock that saving holds.

crates/ide-agent/src/lib.rs

@@ -191,6 +191,8 @@ struct Task {
     dirty: Mutex<BTreeSet<usize>>,
     flush_scheduled: AtomicBool,
     stderr: Mutex<VecDeque<String>>,
+    /// Set by `delete`; held while saving, so a late save can't bring the file back.
+    deleted: Mutex<bool>,
 }
 
 impl AgentManager {
@@ -519,6 +521,7 @@ impl AgentManager {
             blocking(move || worktree::remove(&root, &cwd)).await?;
         }
         self.tasks.write().remove(id);
+        *task.deleted.lock() = true;
         let _ = std::fs::remove_file(self.shared.paths.store_dir.join(format!("{id}.json")));
         self.shared.emit(AgentEvent::Removed { id: id.to_owned() });
         Ok(())
@@ -568,6 +571,7 @@ impl Task {
             dirty: Mutex::new(BTreeSet::new()),
             flush_scheduled: AtomicBool::new(false),
             stderr: Mutex::new(VecDeque::new()),
+            deleted: Mutex::new(false),
         })
     }
 
@@ -604,6 +608,10 @@ impl Task {
             })
         };
         let path = self.shared.paths.store_dir.join(format!("{}.json", self.id));
+        let deleted = self.deleted.lock();
+        if *deleted {
+            return;
+        }
         if let Err(e) = bytes.map_err(|e| e.to_string()).and_then(|b| write_file(&path, &b)) {
             tracing::warn!("saving agent task {}: {e}", self.id);
         }