✨ 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>

adr/README.md

@@ -71,7 +71,7 @@ Chosen option: "[Option 1]" because [justification]
 * [0010-api-v2-feature-flag.md](0010-api-v2-feature-flag.md) - API v2 implementation with feature flag control
 * [0011-validation-library-selection.md](0011-validation-library-selection.md) - Selection of go-playground/validator for input validation
 * [0012-git-hooks-staged-only-formatting.md](0012-git-hooks-staged-only-formatting.md) - Git hooks format only staged Go files
-* [0013-openapi-swagger-toolchain.md](0013-openapi-swagger-toolchain.md) - OpenAPI/Swagger toolchain selection
+* [0013-openapi-swagger-toolchain.md](0013-openapi-swagger-toolchain.md) - ✅ OpenAPI/Swagger documentation with swaggo/swag (Implemented)
 * [0014-grpc-adoption-strategy.md](0014-grpc-adoption-strategy.md) - Hybrid REST/gRPC adoption strategy
 
 ## How to Add a New ADR