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/cpp/string/string_lower/package.json

@@ -0,0 +1,13 @@
+{
+  "name": "@metabuilder/string_lower",
+  "version": "1.0.0",
+  "description": "Convert string to lowercase",
+  "author": "MetaBuilder",
+  "license": "MIT",
+  "keywords": ["string", "workflow", "plugin"],
+  "main": "string_lower.hpp",
+  "metadata": {
+    "plugin_type": "string.lower",
+    "category": "string"
+  }
+}