subagent daemon: pin that an interrupted ACP turn frees its name at once

The ACP interrupt test now asserts the name is not left Stopping; claude's
turns stay Stopping until the process exits, as the claude tests already
assert.

hive-subagent-mcp/src/session.rs

@@ -5102,6 +5102,11 @@ echo '{"type":"result","subtype":"success","is_error":false}'
         .await;
 
         interrupt(&state, "n").expect("a running ACP turn is interruptible");
+        assert_eq!(
+            state.occupancy("n"),
+            None,
+            "an interrupted ACP turn frees its name at once; only claude's stays claimed"
+        );
         until("the run ends", || state.last_event_age("n").is_none()).await;
 
         assert_eq!(lines(&dir.join("prompts")).len(), 1, "no second goal turn");