✨ feat: implement OpenAPI/Swagger documentation with swaggo/swag

📝 docs: add comprehensive API documentation
📦 dependencies: add swaggo/swag to go.mod
🔧 chore: add go:generate directive for documentation

- Add comprehensive API documentation using swaggo/swag
- Embed OpenAPI spec in binary using go:embed
- Add Swagger UI at /swagger/
- Document all endpoints, models, and validation rules
- Add go:generate directive for easy regeneration
- Update README, AGENTS, CHANGELOG with documentation
- Finalize ADR 0013 with implementation details
- Gitignore generated docs directory

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

README.md

@@ -13,6 +13,7 @@ A Go project demonstrating idiomatic package structure, CLI implementation, and
 - Graceful shutdown with context
 - Readiness endpoint for Kubernetes/service mesh integration
 - OpenTelemetry integration with Jaeger support
+- OpenAPI/Swagger documentation
 - Unit tests
 - Go 1.26.1 compatible
 
@@ -76,6 +77,10 @@ go run ./cmd/greet John
 # Test API endpoints
 ./scripts/start-server.sh test
 
+# Access OpenAPI documentation
+# Swagger UI: http://localhost:8080/swagger/
+# OpenAPI spec: http://localhost:8080/swagger/doc.json
+
 # Stop the server
 ./scripts/start-server.sh stop
 ```
@@ -117,6 +122,7 @@ DanceLessonsCoach/
 ├── adr/                    # Architecture Decision Records
 ├── cmd/                    # Entry points (greet CLI, server)
 ├── pkg/                    # Core packages (config, greet, server, telemetry)
+│   └── server/docs/        # Generated OpenAPI documentation (gitignored)
 ├── config.yaml             # Configuration file
 ├── scripts/                # Management scripts
 └── go.mod                   # Go module definition
@@ -125,9 +131,46 @@ DanceLessonsCoach/
 **See [AGENTS.md](AGENTS.md#project-structure) for detailed structure and component explanations.**
 ```
 
+## Development
+
+### Generate OpenAPI Documentation
+
+The project uses [swaggo/swag](https://github.com/swaggo/swag) to generate OpenAPI/Swagger documentation from code annotations:
+
+```bash
+# Generate documentation
+go generate ./pkg/server/
+
+# This creates:
+# - pkg/server/docs/docs.go (swagger template)
+# - pkg/server/docs/swagger.json (OpenAPI spec)
+# - pkg/server/docs/swagger.yaml (YAML version)
+```
+
+**Note:** `pkg/server/docs/` is gitignored. Documentation is embedded in the binary at build time.
+
+### Documentation Annotations
+
+Add swagger annotations to handlers and models:
+
+```go
+// @Summary Get personalized greeting
+// @Description Returns a greeting with the specified name
+// @Tags greet
+// @Accept json
+// @Produce json
+// @Param name path string true "Name to greet"
+// @Success 200 {object} GreetResponse "Successful response"
+// @Failure 400 {object} ErrorResponse "Invalid name parameter"
+// @Router /v1/greet/{name} [get]
+func (h *apiV1GreetHandler) handleGreetPath(w http.ResponseWriter, r *http.Request) {
+    // handler implementation
+}
+```
+
 ## Architecture
 
-This project uses Architecture Decision Records (ADRs) to document key technical choices. See [adr/](adr/) for complete documentation including decisions on Go 1.26.1, Chi router, Zerolog, OpenTelemetry, interface-based design, graceful shutdown, configuration management, and testing strategies.
+This project uses Architecture Decision Records (ADRs) to document key technical choices. See [adr/](adr/) for complete documentation including decisions on Go 1.26.1, Chi router, Zerolog, OpenTelemetry, interface-based design, graceful shutdown, configuration management, testing strategies, and OpenAPI documentation.
 
 **Adding new decisions?** See [adr/README.md](adr/README.md) for guidelines.