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

examples/go-microservice-CLAUDE.md

@@ -0,0 +1,267 @@
+# Go Microservice — Project CLAUDE.md
+
+> Real-world example for a Go microservice with PostgreSQL, gRPC, and Docker.
+> Copy this to your project root and customize for your service.
+
+## Project Overview
+
+**Stack:** Go 1.22+, PostgreSQL, gRPC + REST (grpc-gateway), Docker, sqlc (type-safe SQL), Wire (dependency injection)
+
+**Architecture:** Clean architecture with domain, repository, service, and handler layers. gRPC as primary transport with REST gateway for external clients.
+
+## Critical Rules
+
+### Go Conventions
+
+- Follow Effective Go and the Go Code Review Comments guide
+- Use `errors.New` / `fmt.Errorf` with `%w` for wrapping — never string matching on errors
+- No `init()` functions — explicit initialization in `main()` or constructors
+- No global mutable state — pass dependencies via constructors
+- Context must be the first parameter and propagated through all layers
+
+### Database
+
+- All queries in `queries/` as plain SQL — sqlc generates type-safe Go code
+- Migrations in `migrations/` using golang-migrate — never alter the database directly
+- Use transactions for multi-step operations via `pgx.Tx`
+- All queries must use parameterized placeholders (`$1`, `$2`) — never string formatting
+
+### Error Handling
+
+- Return errors, don't panic — panics are only for truly unrecoverable situations
+- Wrap errors with context: `fmt.Errorf("creating user: %w", err)`
+- Define sentinel errors in `domain/errors.go` for business logic
+- Map domain errors to gRPC status codes in the handler layer
+
+```go
+// Domain layer — sentinel errors
+var (
+    ErrUserNotFound  = errors.New("user not found")
+    ErrEmailTaken    = errors.New("email already registered")
+)
+
+// Handler layer — map to gRPC status
+func toGRPCError(err error) error {
+    switch {
+    case errors.Is(err, domain.ErrUserNotFound):
+        return status.Error(codes.NotFound, err.Error())
+    case errors.Is(err, domain.ErrEmailTaken):
+        return status.Error(codes.AlreadyExists, err.Error())
+    default:
+        return status.Error(codes.Internal, "internal error")
+    }
+}
+```
+
+### Code Style
+
+- No emojis in code or comments
+- Exported types and functions must have doc comments
+- Keep functions under 50 lines — extract helpers
+- Use table-driven tests for all logic with multiple cases
+- Prefer `struct{}` for signal channels, not `bool`
+
+## File Structure
+
+```
+cmd/
+  server/
+    main.go              # Entrypoint, Wire injection, graceful shutdown
+internal/
+  domain/                # Business types and interfaces
+    user.go              # User entity and repository interface
+    errors.go            # Sentinel errors
+  service/               # Business logic
+    user_service.go
+    user_service_test.go
+  repository/            # Data access (sqlc-generated + custom)
+    postgres/
+      user_repo.go
+      user_repo_test.go  # Integration tests with testcontainers
+  handler/               # gRPC + REST handlers
+    grpc/
+      user_handler.go
+    rest/
+      user_handler.go
+  config/                # Configuration loading
+    config.go
+proto/                   # Protobuf definitions
+  user/v1/
+    user.proto
+queries/                 # SQL queries for sqlc
+  user.sql
+migrations/              # Database migrations
+  001_create_users.up.sql
+  001_create_users.down.sql
+```
+
+## Key Patterns
+
+### Repository Interface
+
+```go
+type UserRepository interface {
+    Create(ctx context.Context, user *User) error
+    FindByID(ctx context.Context, id uuid.UUID) (*User, error)
+    FindByEmail(ctx context.Context, email string) (*User, error)
+    Update(ctx context.Context, user *User) error
+    Delete(ctx context.Context, id uuid.UUID) error
+}
+```
+
+### Service with Dependency Injection
+
+```go
+type UserService struct {
+    repo   domain.UserRepository
+    hasher PasswordHasher
+    logger *slog.Logger
+}
+
+func NewUserService(repo domain.UserRepository, hasher PasswordHasher, logger *slog.Logger) *UserService {
+    return &UserService{repo: repo, hasher: hasher, logger: logger}
+}
+
+func (s *UserService) Create(ctx context.Context, req CreateUserRequest) (*domain.User, error) {
+    existing, err := s.repo.FindByEmail(ctx, req.Email)
+    if err != nil && !errors.Is(err, domain.ErrUserNotFound) {
+        return nil, fmt.Errorf("checking email: %w", err)
+    }
+    if existing != nil {
+        return nil, domain.ErrEmailTaken
+    }
+
+    hashed, err := s.hasher.Hash(req.Password)
+    if err != nil {
+        return nil, fmt.Errorf("hashing password: %w", err)
+    }
+
+    user := &domain.User{
+        ID:       uuid.New(),
+        Name:     req.Name,
+        Email:    req.Email,
+        Password: hashed,
+    }
+    if err := s.repo.Create(ctx, user); err != nil {
+        return nil, fmt.Errorf("creating user: %w", err)
+    }
+    return user, nil
+}
+```
+
+### Table-Driven Tests
+
+```go
+func TestUserService_Create(t *testing.T) {
+    tests := []struct {
+        name    string
+        req     CreateUserRequest
+        setup   func(*MockUserRepo)
+        wantErr error
+    }{
+        {
+            name: "valid user",
+            req:  CreateUserRequest{Name: "Alice", Email: "alice@example.com", Password: "secure123"},
+            setup: func(m *MockUserRepo) {
+                m.On("FindByEmail", mock.Anything, "alice@example.com").Return(nil, domain.ErrUserNotFound)
+                m.On("Create", mock.Anything, mock.Anything).Return(nil)
+            },
+            wantErr: nil,
+        },
+        {
+            name: "duplicate email",
+            req:  CreateUserRequest{Name: "Alice", Email: "taken@example.com", Password: "secure123"},
+            setup: func(m *MockUserRepo) {
+                m.On("FindByEmail", mock.Anything, "taken@example.com").Return(&domain.User{}, nil)
+            },
+            wantErr: domain.ErrEmailTaken,
+        },
+    }
+
+    for _, tt := range tests {
+        t.Run(tt.name, func(t *testing.T) {
+            repo := new(MockUserRepo)
+            tt.setup(repo)
+            svc := NewUserService(repo, &bcryptHasher{}, slog.Default())
+
+            _, err := svc.Create(context.Background(), tt.req)
+
+            if tt.wantErr != nil {
+                assert.ErrorIs(t, err, tt.wantErr)
+            } else {
+                assert.NoError(t, err)
+            }
+        })
+    }
+}
+```
+
+## Environment Variables
+
+```bash
+# Database
+DATABASE_URL=postgres://user:pass@localhost:5432/myservice?sslmode=disable
+
+# gRPC
+GRPC_PORT=50051
+REST_PORT=8080
+
+# Auth
+JWT_SECRET=           # Load from vault in production
+TOKEN_EXPIRY=24h
+
+# Observability
+LOG_LEVEL=info        # debug, info, warn, error
+OTEL_ENDPOINT=        # OpenTelemetry collector
+```
+
+## Testing Strategy
+
+```bash
+/go-test             # TDD workflow for Go
+/go-review           # Go-specific code review
+/go-build            # Fix build errors
+```
+
+### Test Commands
+
+```bash
+# Unit tests (fast, no external deps)
+go test ./internal/... -short -count=1
+
+# Integration tests (requires Docker for testcontainers)
+go test ./internal/repository/... -count=1 -timeout 120s
+
+# All tests with coverage
+go test ./... -coverprofile=coverage.out -count=1
+go tool cover -func=coverage.out  # summary
+go tool cover -html=coverage.out  # browser
+
+# Race detector
+go test ./... -race -count=1
+```
+
+## ECC Workflow
+
+```bash
+# Planning
+/plan "Add rate limiting to user endpoints"
+
+# Development
+/go-test                  # TDD with Go-specific patterns
+
+# Review
+/go-review                # Go idioms, error handling, concurrency
+/security-scan            # Secrets and vulnerabilities
+
+# Before merge
+go vet ./...
+staticcheck ./...
+```
+
+## Git Workflow
+
+- `feat:` new features, `fix:` bug fixes, `refactor:` code changes
+- Feature branches from `main`, PRs required
+- CI: `go vet`, `staticcheck`, `go test -race`, `golangci-lint`
+- Deploy: Docker image built in CI, deployed to Kubernetes