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_length/string_length.hpp

@@ -0,0 +1,15 @@
+#pragma once
+/**
+ * Workflow plugin: get string length
+ */
+
+#include "../../plugin.hpp"
+
+namespace metabuilder::workflow::string {
+
+inline PluginResult length(Runtime&, const json& inputs) {
+    auto str = inputs.value("string", std::string{});
+    return {{"result", str.length()}};
+}
+
+} // namespace metabuilder::workflow::string