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_concat/string_concat.go

@@ -1,18 +1,32 @@
-// Package string_concat provides the concatenate strings plugin.
+// Package string_concat provides a workflow plugin for concatenating strings.
 package string_concat
 
 import (
 	"fmt"
 	"strings"
-
-	plugin "metabuilder/workflow/plugins/go"
 )
 
-// Run concatenates multiple strings with optional separator.
-func Run(runtime *plugin.Runtime, inputs map[string]interface{}) (map[string]interface{}, error) {
+// StringConcat implements the NodeExecutor interface for concatenating strings.
+type StringConcat struct {
+	NodeType    string
+	Category    string
+	Description string
+}
+
+// NewStringConcat creates a new StringConcat instance.
+func NewStringConcat() *StringConcat {
+	return &StringConcat{
+		NodeType:    "string.concat",
+		Category:    "string",
+		Description: "Concatenate multiple strings with optional separator",
+	}
+}
+
+// Execute runs the plugin logic.
+func (p *StringConcat) Execute(inputs map[string]interface{}, runtime interface{}) map[string]interface{} {
 	strs, ok := inputs["strings"].([]interface{})
 	if !ok {
-		return map[string]interface{}{"result": "", "error": "strings must be an array"}, nil
+		return map[string]interface{}{"result": "", "error": "strings must be an array"}
 	}
 
 	separator := ""
@@ -25,5 +39,5 @@ func Run(runtime *plugin.Runtime, inputs map[string]interface{}) (map[string]int
 		strList[i] = fmt.Sprintf("%v", s)
 	}
 
-	return map[string]interface{}{"result": strings.Join(strList, separator)}, nil
+	return map[string]interface{}{"result": strings.Join(strList, separator)}
 }