feat(hello): implement PlanForce placeholder to rewrite marker

internal/modules/hello/hello_test.go

@@ -55,3 +55,31 @@ func TestHelloFullLifecycle(t *testing.T) {
 		t.Fatalf("content = %q", data)
 	}
 }
+
+func TestHelloPlanForceNonEmpty(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "hello.ok")
+	m := Module{Path: path}
+	// Even after marker is present, force plan must run.
+	if err := os.WriteFile(path, []byte(markerContent), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	plan, err := m.PlanForce(context.Background())
+	if err != nil {
+		t.Fatalf("PlanForce: %v", err)
+	}
+	if plan.IsEmpty() {
+		t.Fatal("force plan must not be empty")
+	}
+	for _, a := range plan.Actions {
+		if err := a.Exec(context.Background()); err != nil {
+			t.Fatalf("Exec: %v", err)
+		}
+	}
+	data, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(data) != markerContent {
+		t.Fatalf("content after force = %q", data)
+	}
+}