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_trim/package.json

@@ -0,0 +1,13 @@
+{
+  "name": "@metabuilder/string_trim",
+  "version": "1.0.0",
+  "description": "Trim whitespace from string",
+  "author": "MetaBuilder",
+  "license": "MIT",
+  "keywords": ["string", "workflow", "plugin"],
+  "main": "string_trim.hpp",
+  "metadata": {
+    "plugin_type": "string.trim",
+    "category": "string"
+  }
+}

workflow/plugins/cpp/string/string_trim/string_trim.hpp

@@ -0,0 +1,42 @@
+#pragma once
+/**
+ * Workflow plugin: trim whitespace from string
+ */
+
+#include "../../plugin.hpp"
+#include <algorithm>
+#include <cctype>
+
+namespace metabuilder::workflow::string {
+
+inline PluginResult trim(Runtime&, const json& inputs) {
+    auto text = inputs.value("text", std::string{});
+    auto mode = inputs.value("mode", std::string{"both"});
+
+    auto ltrim = [](std::string& s) {
+        s.erase(s.begin(), std::find_if(s.begin(), s.end(), [](unsigned char ch) {
+            return !std::isspace(ch);
+        }));
+    };
+
+    auto rtrim = [](std::string& s) {
+        s.erase(std::find_if(s.rbegin(), s.rend(), [](unsigned char ch) {
+            return !std::isspace(ch);
+        }).base(), s.end());
+    };
+
+    std::string result = text;
+
+    if (mode == "start") {
+        ltrim(result);
+    } else if (mode == "end") {
+        rtrim(result);
+    } else {
+        ltrim(result);
+        rtrim(result);
+    }
+
+    return {{"result", result}};
+}
+
+} // namespace metabuilder::workflow::string