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_replace/string_replace.hpp

@@ -0,0 +1,34 @@
+#pragma once
+/**
+ * Workflow plugin: replace occurrences in string
+ */
+
+#include "../../plugin.hpp"
+
+namespace metabuilder::workflow::string {
+
+inline PluginResult replace(Runtime&, const json& inputs) {
+    auto text = inputs.value("text", std::string{});
+    auto old_str = inputs.value("old", std::string{});
+    auto new_str = inputs.value("new", std::string{});
+    auto count = inputs.contains("count") ? inputs["count"].get<int>() : -1;
+
+    if (old_str.empty()) {
+        return {{"result", text}};
+    }
+
+    std::string result = text;
+    size_t pos = 0;
+    int replaced = 0;
+
+    while ((pos = result.find(old_str, pos)) != std::string::npos) {
+        if (count >= 0 && replaced >= count) break;
+        result.replace(pos, old_str.length(), new_str);
+        pos += new_str.length();
+        ++replaced;
+    }
+
+    return {{"result", result}};
+}
+
+} // namespace metabuilder::workflow::string