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/python/string/string_concat/factory.py

@@ -0,0 +1,7 @@
+"""Factory for StringConcat plugin."""
+
+from .string_concat import StringConcat
+
+
+def create():
+    return StringConcat()

workflow/plugins/python/string/string_concat/package.json

@@ -1,13 +1,16 @@
 {
   "name": "@metabuilder/string_concat",
   "version": "1.0.0",
-  "description": "string_concat plugin",
+  "description": "Concatenate multiple strings",
   "author": "MetaBuilder",
   "license": "MIT",
   "keywords": ["string", "workflow", "plugin"],
   "main": "string_concat.py",
+  "files": ["string_concat.py", "factory.py"],
   "metadata": {
     "plugin_type": "string.concat",
-    "category": "string"
+    "category": "string",
+    "class": "StringConcat",
+    "entrypoint": "execute"
   }
 }

workflow/plugins/python/string/string_concat/string_concat.py

@@ -1,10 +1,16 @@
 """Workflow plugin: concatenate strings."""
 
+from ...base import NodeExecutor
 
-def run(_runtime, inputs):
+
+class StringConcat(NodeExecutor):
     """Concatenate multiple strings."""
-    strings = inputs.get("strings", [])
-    separator = inputs.get("separator", "")
 
-    str_list = [str(s) for s in strings]
-    return {"result": separator.join(str_list)}
+    node_type = "string.concat"
+    category = "string"
+    description = "Concatenate multiple strings"
+
+    def execute(self, inputs, runtime=None):
+        separator = inputs.get("separator", "")
+        strings = inputs.get("strings", inputs.get("values", []))
+        return {"result": separator.join(str(s) for s in strings)}