feat: add LLM step executor with vision and tool support, event workflow system, and inheritance

- Add LLM executor supporting OpenAI vision, tool calling, embeddings, and structured outputs
- Introduce event emitter/receiver workflows with deduplication and filtering (generate_event functions)
- Add workflow extends/override system enabling inheritance chains and step merge modes
- Update function naming to snake_case across all testdata (fileExists→file_exists, etc.)
- Add comprehensive test fixtures for linter, events, CDN, step dependencies, and extends workflows

internal/executor/function_executor.go

@@ -13,12 +13,12 @@ import (
 
 // FunctionExecutor executes function steps
 type FunctionExecutor struct {
-	templateEngine   *template.Engine
+	templateEngine   template.TemplateEngine
 	functionRegistry *functions.Registry
 }
 
 // NewFunctionExecutor creates a new function executor
-func NewFunctionExecutor(engine *template.Engine, registry *functions.Registry) *FunctionExecutor {
+func NewFunctionExecutor(engine template.TemplateEngine, registry *functions.Registry) *FunctionExecutor {
 	return &FunctionExecutor{
 		templateEngine:   engine,
 		functionRegistry: registry,