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/package.json

@@ -0,0 +1,13 @@
+{
+  "name": "@metabuilder/string_replace",
+  "version": "1.0.0",
+  "description": "Replace occurrences in string",
+  "author": "MetaBuilder",
+  "license": "MIT",
+  "keywords": ["string", "workflow", "plugin"],
+  "main": "string_replace.hpp",
+  "metadata": {
+    "plugin_type": "string.replace",
+    "category": "string"
+  }
+}