add find by user method in plan repo

app/Plan/JsonPlanRepository.php

@@ -90,4 +90,29 @@ class JsonPlanRepository implements PlanRepository
 
         return $maxId + 1;
     }
+
+    public function findByUser(User $user): array
+    {
+        $plans = array_filter(
+            $this->readPlans(),
+            function ($data) use ($user) {
+                return $data['userId'] === $user->getId();
+            }
+        );
+
+        return array_map(
+            function ($data) {
+                $user = $this->userRepository->find($data['userId']);
+                if ($user === null) {
+                    return null;
+                }
+                return new Plan(
+                    id: $data['id'],
+                    name: $data['name'],
+                    user: $user,
+                );
+            },
+            $plans
+        );
+    }
 }