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/package.json

@@ -0,0 +1,22 @@
+{
+  "name": "@metabuilder/workflow-plugins-string",
+  "version": "1.0.0",
+  "description": "String manipulation plugins",
+  "author": "MetaBuilder",
+  "license": "MIT",
+  "keywords": ["string", "workflow", "plugins"],
+  "metadata": {
+    "category": "string",
+    "plugin_count": 8
+  },
+  "plugins": [
+    "string_concat",
+    "string_format",
+    "string_length",
+    "string_lower",
+    "string_replace",
+    "string_split",
+    "string_trim",
+    "string_upper"
+  ]
+}