feat(rules): add Java language rules (#645)

Adds Java language rules (coding-style, hooks, patterns, security, testing) following the established language rule conventions.

rules/java/testing.md

@@ -0,0 +1,131 @@
+---
+paths:
+  - "**/*.java"
+---
+# Java Testing
+
+> This file extends [common/testing.md](../common/testing.md) with Java-specific content.
+
+## Test Framework
+
+- **JUnit 5** (`@Test`, `@ParameterizedTest`, `@Nested`, `@DisplayName`)
+- **AssertJ** for fluent assertions (`assertThat(result).isEqualTo(expected)`)
+- **Mockito** for mocking dependencies
+- **Testcontainers** for integration tests requiring databases or services
+
+## Test Organization
+
+```
+src/test/java/com/example/app/
+  service/           # Unit tests for service layer
+  controller/        # Web layer / API tests
+  repository/        # Data access tests
+  integration/       # Cross-layer integration tests
+```
+
+Mirror the `src/main/java` package structure in `src/test/java`.
+
+## Unit Test Pattern
+
+```java
+@ExtendWith(MockitoExtension.class)
+class OrderServiceTest {
+
+    @Mock
+    private OrderRepository orderRepository;
+
+    private OrderService orderService;
+
+    @BeforeEach
+    void setUp() {
+        orderService = new OrderService(orderRepository);
+    }
+
+    @Test
+    @DisplayName("findById returns order when exists")
+    void findById_existingOrder_returnsOrder() {
+        var order = new Order(1L, "Alice", BigDecimal.TEN);
+        when(orderRepository.findById(1L)).thenReturn(Optional.of(order));
+
+        var result = orderService.findById(1L);
+
+        assertThat(result.customerName()).isEqualTo("Alice");
+        verify(orderRepository).findById(1L);
+    }
+
+    @Test
+    @DisplayName("findById throws when order not found")
+    void findById_missingOrder_throws() {
+        when(orderRepository.findById(99L)).thenReturn(Optional.empty());
+
+        assertThatThrownBy(() -> orderService.findById(99L))
+            .isInstanceOf(OrderNotFoundException.class)
+            .hasMessageContaining("99");
+    }
+}
+```
+
+## Parameterized Tests
+
+```java
+@ParameterizedTest
+@CsvSource({
+    "100.00, 10, 90.00",
+    "50.00, 0, 50.00",
+    "200.00, 25, 150.00"
+})
+@DisplayName("discount applied correctly")
+void applyDiscount(BigDecimal price, int pct, BigDecimal expected) {
+    assertThat(PricingUtils.discount(price, pct)).isEqualByComparingTo(expected);
+}
+```
+
+## Integration Tests
+
+Use Testcontainers for real database integration:
+
+```java
+@Testcontainers
+class OrderRepositoryIT {
+
+    @Container
+    static PostgreSQLContainer<?> postgres = new PostgreSQLContainer<>("postgres:16");
+
+    private OrderRepository repository;
+
+    @BeforeEach
+    void setUp() {
+        var dataSource = new PGSimpleDataSource();
+        dataSource.setUrl(postgres.getJdbcUrl());
+        dataSource.setUser(postgres.getUsername());
+        dataSource.setPassword(postgres.getPassword());
+        repository = new JdbcOrderRepository(dataSource);
+    }
+
+    @Test
+    void save_and_findById() {
+        var saved = repository.save(new Order(null, "Bob", BigDecimal.ONE));
+        var found = repository.findById(saved.getId());
+        assertThat(found).isPresent();
+    }
+}
+```
+
+For Spring Boot integration tests, see skill: `springboot-tdd`.
+
+## Test Naming
+
+Use descriptive names with `@DisplayName`:
+- `methodName_scenario_expectedBehavior()` for method names
+- `@DisplayName("human-readable description")` for reports
+
+## Coverage
+
+- Target 80%+ line coverage
+- Use JaCoCo for coverage reporting
+- Focus on service and domain logic — skip trivial getters/config classes
+
+## References
+
+See skill: `springboot-tdd` for Spring Boot TDD patterns with MockMvc and Testcontainers.
+See skill: `java-coding-standards` for testing expectations.