feat: Propagate workflow plugins to Go, Rust, C++, and Mojo

Multi-language workflow plugin system following Python's structure:
- Each plugin in its own directory with implementation + package.json
- Category-level package.json manifests listing all plugins
- Consistent interface: run(runtime, inputs) -> outputs

Languages added:
- Go: math, string, logic, list, dict, var, convert (25+ plugins)
- Rust: math, string, logic, list, convert, var (50+ functions)
- C++: header-only math, string, logic, var, convert (30+ plugins)
- Mojo: math, string, list with systems programming features

Python structure fixed:
- Reorganized flat files into plugin subdirectories
- Added package.json metadata to all 120+ plugins
- Added missing backend category (15 plugins)
- Category manifests with plugin lists

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

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

@@ -0,0 +1,13 @@
+{
+  "name": "@metabuilder/string_split",
+  "version": "1.0.0",
+  "description": "string_split plugin",
+  "author": "MetaBuilder",
+  "license": "MIT",
+  "keywords": ["string", "workflow", "plugin"],
+  "main": "string_split.py",
+  "metadata": {
+    "plugin_type": "string.split",
+    "category": "string"
+  }
+}

workflow/plugins/python/string/string_split/string_split.py

@@ -0,0 +1,15 @@
+"""Workflow plugin: split string."""
+
+
+def run(_runtime, inputs):
+    """Split string by separator."""
+    text = inputs.get("text", "")
+    separator = inputs.get("separator", " ")
+    max_splits = inputs.get("max_splits")
+
+    if max_splits is not None:
+        result = text.split(separator, max_splits)
+    else:
+        result = text.split(separator)
+
+    return {"result": result}