docs: expand Spring Boot skills and add Go microservice example

- springboot-security: add code examples for authorization, input validation,
  SQL injection prevention, password encoding, CORS, rate limiting, and secrets
  management (119 → 261 lines)
- springboot-verification: add unit test, Testcontainers integration test,
  MockMvc API test patterns, and security scan grep commands (100 → 222 lines)
- Add Go microservice example (gRPC + PostgreSQL + clean architecture)
- Update README directory tree with new example

skills/springboot-verification/SKILL.md

@@ -42,6 +42,111 @@ Report:
 - Total tests, passed/failed
 - Coverage % (lines/branches)
 
+### Unit Tests
+
+Test service logic in isolation with mocked dependencies:
+
+```java
+@ExtendWith(MockitoExtension.class)
+class UserServiceTest {
+
+  @Mock private UserRepository userRepository;
+  @InjectMocks private UserService userService;
+
+  @Test
+  void createUser_validInput_returnsUser() {
+    var dto = new CreateUserDto("Alice", "alice@example.com");
+    var expected = new User(1L, "Alice", "alice@example.com");
+    when(userRepository.save(any(User.class))).thenReturn(expected);
+
+    var result = userService.create(dto);
+
+    assertThat(result.name()).isEqualTo("Alice");
+    verify(userRepository).save(any(User.class));
+  }
+
+  @Test
+  void createUser_duplicateEmail_throwsException() {
+    var dto = new CreateUserDto("Alice", "existing@example.com");
+    when(userRepository.existsByEmail(dto.email())).thenReturn(true);
+
+    assertThatThrownBy(() -> userService.create(dto))
+        .isInstanceOf(DuplicateEmailException.class);
+  }
+}
+```
+
+### Integration Tests with Testcontainers
+
+Test against a real database instead of H2:
+
+```java
+@SpringBootTest
+@Testcontainers
+class UserRepositoryIntegrationTest {
+
+  @Container
+  static PostgreSQLContainer<?> postgres = new PostgreSQLContainer<>("postgres:16-alpine")
+      .withDatabaseName("testdb");
+
+  @DynamicPropertySource
+  static void configureProperties(DynamicPropertyRegistry registry) {
+    registry.add("spring.datasource.url", postgres::getJdbcUrl);
+    registry.add("spring.datasource.username", postgres::getUsername);
+    registry.add("spring.datasource.password", postgres::getPassword);
+  }
+
+  @Autowired private UserRepository userRepository;
+
+  @Test
+  void findByEmail_existingUser_returnsUser() {
+    userRepository.save(new User("Alice", "alice@example.com"));
+
+    var found = userRepository.findByEmail("alice@example.com");
+
+    assertThat(found).isPresent();
+    assertThat(found.get().getName()).isEqualTo("Alice");
+  }
+}
+```
+
+### API Tests with MockMvc
+
+Test controller layer with full Spring context:
+
+```java
+@WebMvcTest(UserController.class)
+class UserControllerTest {
+
+  @Autowired private MockMvc mockMvc;
+  @MockBean private UserService userService;
+
+  @Test
+  void createUser_validInput_returns201() throws Exception {
+    var user = new UserDto(1L, "Alice", "alice@example.com");
+    when(userService.create(any())).thenReturn(user);
+
+    mockMvc.perform(post("/api/users")
+            .contentType(MediaType.APPLICATION_JSON)
+            .content("""
+                {"name": "Alice", "email": "alice@example.com"}
+                """))
+        .andExpect(status().isCreated())
+        .andExpect(jsonPath("$.name").value("Alice"));
+  }
+
+  @Test
+  void createUser_invalidEmail_returns400() throws Exception {
+    mockMvc.perform(post("/api/users")
+            .contentType(MediaType.APPLICATION_JSON)
+            .content("""
+                {"name": "Alice", "email": "not-an-email"}
+                """))
+        .andExpect(status().isBadRequest());
+  }
+}
+```
+
 ## Phase 4: Security Scan
 
 ```bash
@@ -50,10 +155,27 @@ mvn org.owasp:dependency-check-maven:check
 # or
 ./gradlew dependencyCheckAnalyze
 
-# Secrets (git)
+# Secrets in source
+grep -rn "password\s*=\s*\"" src/ --include="*.java" --include="*.yml" --include="*.properties"
+grep -rn "sk-\|api_key\|secret" src/ --include="*.java" --include="*.yml"
+
+# Secrets (git history)
 git secrets --scan  # if configured
 ```
 
+### Common Security Findings
+
+```
+# Check for System.out.println (use logger instead)
+grep -rn "System\.out\.print" src/main/ --include="*.java"
+
+# Check for raw exception messages in responses
+grep -rn "e\.getMessage()" src/main/ --include="*.java"
+
+# Check for wildcard CORS
+grep -rn "allowedOrigins.*\*" src/main/ --include="*.java"
+```
+
 ## Phase 5: Lint/Format (optional gate)
 
 ```bash