refactor: Restructure multi-language workflow plugins to match Python pattern

Each plugin now follows the standardized structure:
- category/plugin_name/plugin_name.{ext} - Implementation
- category/plugin_name/package.json - Plugin metadata
- category/package.json - Category manifest with "plugins" array

Languages restructured:
- Go: 31 plugins across 7 categories (math, string, logic, list, dict, var, convert)
- Rust: 55 plugins as individual crates in Cargo workspace
- C++: 34 header-only plugins with complete coverage
- Mojo: 11 plugins across 3 categories (math, string, list)

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

workflow/plugins/cpp/string/string_split/string_split.hpp

@@ -0,0 +1,39 @@
+#pragma once
+/**
+ * Workflow plugin: split string by separator
+ */
+
+#include "../../plugin.hpp"
+#include <vector>
+#include <sstream>
+
+namespace metabuilder::workflow::string {
+
+inline PluginResult split(Runtime&, const json& inputs) {
+    auto text = inputs.value("text", std::string{});
+    auto separator = inputs.value("separator", std::string{" "});
+    auto max_splits = inputs.contains("max_splits") ? inputs["max_splits"].get<int>() : -1;
+
+    std::vector<std::string> result;
+
+    if (separator.empty()) {
+        result.push_back(text);
+        return {{"result", result}};
+    }
+
+    size_t pos = 0;
+    size_t prev = 0;
+    int count = 0;
+
+    while ((pos = text.find(separator, prev)) != std::string::npos) {
+        if (max_splits >= 0 && count >= max_splits) break;
+        result.push_back(text.substr(prev, pos - prev));
+        prev = pos + separator.length();
+        ++count;
+    }
+    result.push_back(text.substr(prev));
+
+    return {{"result", result}};
+}
+
+} // namespace metabuilder::workflow::string