fix(FN-993): add safe-navigation for task.description in recoverCompletedTasks

- Add optional chaining (?.) on task.description.slice() to prevent null-reference crash
- Fallback to '(untitled)' when both title and description are absent
- Add changeset for @fusion/engine patch

.changeset/fix-recover-completed-task-log.md

@@ -0,0 +1,5 @@
+---
+"@fusion/engine": patch
+---
+
+Fix null-reference error in recoverCompletedTasks when logging tasks without title or description.