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_concat/string_concat.hpp

@@ -0,0 +1,22 @@
+#pragma once
+/**
+ * Workflow plugin: concatenate strings
+ */
+
+#include "../../plugin.hpp"
+
+namespace metabuilder::workflow::string {
+
+inline PluginResult concat(Runtime&, const json& inputs) {
+    auto strings = inputs.value("strings", std::vector<std::string>{});
+    auto separator = inputs.value("separator", std::string{});
+
+    std::string result;
+    for (size_t i = 0; i < strings.size(); ++i) {
+        if (i > 0) result += separator;
+        result += strings[i];
+    }
+    return {{"result", result}};
+}
+
+} // namespace metabuilder::workflow::string