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_length/package.json

@@ -1,13 +1,16 @@
 {
   "name": "@metabuilder/string_length",
   "version": "1.0.0",
-  "description": "string_length plugin",
+  "description": "Get string length",
   "author": "MetaBuilder",
   "license": "MIT",
   "keywords": ["string", "workflow", "plugin"],
   "main": "string_length.py",
+  "files": ["string_length.py", "factory.py"],
   "metadata": {
     "plugin_type": "string.length",
-    "category": "string"
+    "category": "string",
+    "class": "StringLength",
+    "entrypoint": "execute"
   }
 }