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

packages/engine/src/self-healing.ts

@@ -293,7 +293,7 @@ export class SelfHealingManager {
 
       let recovered = 0;
       for (const task of stuckCompleted) {
-        log.log(`Recovering completed task ${task.id}: ${task.title || task.description.slice(0, 60)}`);
+        log.log(`Recovering completed task ${task.id}: ${task.title || task.description?.slice(0, 60) || "(untitled)"}`);
         const success = await recoverFn(task);
         if (success) recovered++;
       }