✨ 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>

pkg/greet/api_v2.go

@@ -44,6 +44,16 @@ type greetResponse struct {
 	Message string `json:"message"`
 }
 
+// handleGreetPost godoc
+// @Summary Get greeting (v2)
+// @Description Returns a greeting message with validation (v2)
+// @Tags greet
+// @Accept json
+// @Produce json
+// @Param request body GreetRequest true "Greeting request"
+// @Success 200 {object} GreetResponseV2 "Successful response"
+// @Failure 400 {object} ValidationError "Validation error"
+// @Router /v2/greet [post]
 func (h *apiV2GreetHandler) handleGreetPost(w http.ResponseWriter, r *http.Request) {
 	// Read request body
 	body, err := io.ReadAll(r.Body)