✨ 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>

AGENTS.md

@@ -346,6 +346,33 @@ curl -s http://localhost:8080/api/health
 http://localhost:8080
 ```
 
+### OpenAPI Documentation
+
+**Swagger UI:** `http://localhost:8080/swagger/`
+**OpenAPI Spec:** `http://localhost:8080/swagger/doc.json`
+
+The API provides interactive documentation using Swagger UI with complete OpenAPI 2.0 specification. All endpoints, request/response models, and validation rules are documented.
+
+**Features:**
+- Interactive API exploration
+- Try-it-out functionality
+- Model schemas with examples
+- Response examples
+- Validation rules documentation
+
+**Generation:** Documentation is auto-generated from code annotations using [swaggo/swag](https://github.com/swaggo/swag).
+
+```bash
+# Generate documentation
+go generate ./pkg/server/
+```
+
+**Embedded Documentation:** The OpenAPI spec is embedded in the binary using Go's `//go:embed` directive for single-binary deployment.
+
+---
+
+### Health Check
+
 ### Health Check
 ```http
 GET /api/health
@@ -819,7 +846,7 @@ defer cancel()
 
 ### Architectural Improvements
 - [ ] Request validation middleware
-- [ ] OpenAPI/Swagger documentation
+- ✅ OpenAPI/Swagger documentation with embedded spec
 - [ ] Enhanced OpenTelemetry instrumentation
 - [ ] Metrics collection and visualization
 - [ ] Health check improvements