refactor(workflow): convert all plugins to class/struct + factory pattern

- Python: class extending NodeExecutor + factory.py (80+ plugins)
- TypeScript: class implements NodeExecutor + factory.ts (7 groups, 116 classes)
- Go: struct with methods + factory.go (36 plugins)
- Rust: struct impl NodeExecutor trait + factory.rs (54 plugins)
- Mojo: struct + factory.mojo (11 plugins)

All package.json files now include:
- files array listing source files
- metadata.class/struct field
- metadata.entrypoint field

This enables a unified plugin loading system across all languages
with no import side effects (Spring-style DI pattern).

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

workflow/plugins/go/string/string_lower/factory.go

@@ -0,0 +1,7 @@
+// Package string_lower provides factory for StringLower plugin.
+package string_lower
+
+// Create returns a new StringLower instance.
+func Create() *StringLower {
+	return NewStringLower()
+}

workflow/plugins/go/string/string_lower/package.json

@@ -6,8 +6,11 @@
   "license": "MIT",
   "keywords": ["string", "workflow", "plugin"],
   "main": "string_lower.go",
+  "files": ["string_lower.go", "factory.go"],
   "metadata": {
     "plugin_type": "string.lower",
-    "category": "string"
+    "category": "string",
+    "struct": "StringLower",
+    "entrypoint": "Execute"
   }
 }

workflow/plugins/go/string/string_lower/string_lower.go

@@ -1,18 +1,32 @@
-// Package string_lower provides the lowercase string plugin.
+// Package string_lower provides a workflow plugin for lowercasing strings.
 package string_lower
 
 import (
 	"strings"
-
-	plugin "metabuilder/workflow/plugins/go"
 )
 
-// Run converts string to lowercase.
-func Run(runtime *plugin.Runtime, inputs map[string]interface{}) (map[string]interface{}, error) {
+// StringLower implements the NodeExecutor interface for lowercasing strings.
+type StringLower struct {
+	NodeType    string
+	Category    string
+	Description string
+}
+
+// NewStringLower creates a new StringLower instance.
+func NewStringLower() *StringLower {
+	return &StringLower{
+		NodeType:    "string.lower",
+		Category:    "string",
+		Description: "Convert string to lowercase",
+	}
+}
+
+// Execute runs the plugin logic.
+func (p *StringLower) Execute(inputs map[string]interface{}, runtime interface{}) map[string]interface{} {
 	str, ok := inputs["string"].(string)
 	if !ok {
-		return map[string]interface{}{"result": "", "error": "string is required"}, nil
+		return map[string]interface{}{"result": "", "error": "string is required"}
 	}
 
-	return map[string]interface{}{"result": strings.ToLower(str)}, nil
+	return map[string]interface{}{"result": strings.ToLower(str)}
 }