feat(features): contract suites for all repository interfaces

Each repository interface now has a contract suite under
src/__contracts__/. Both Mock and Payload implementations run the
same suite, eliminating mock-vs-real drift. Payload impls back the
contract with an in-memory stub via vi.mock('payload') + a small
buildPayloadStub helper.

Spec: §5.2, §6.4

packages/auth/src/infrastructure/repositories/mock-users.repository.test.ts

@@ -0,0 +1,8 @@
+import { describe } from "vitest";
+import { MockUsersRepository } from "@/infrastructure/repositories/mock-users.repository";
+import { usersRepositoryContract } from "@/__contracts__/users-repository.contract";
+
+describe("MockUsersRepository", () => {
+  // Start with empty store so contract tests run from a clean slate.
+  usersRepositoryContract.run(() => new MockUsersRepository([]));
+});