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/go/string/string_replace/string_replace.go

@@ -0,0 +1,28 @@
+// Package string_replace provides the replace string plugin.
+package string_replace
+
+import (
+	"strings"
+
+	plugin "metabuilder/workflow/plugins/go"
+)
+
+// Run replaces occurrences in a string.
+func Run(runtime *plugin.Runtime, inputs map[string]interface{}) (map[string]interface{}, error) {
+	str, ok := inputs["string"].(string)
+	if !ok {
+		return map[string]interface{}{"result": "", "error": "string is required"}, nil
+	}
+
+	old, _ := inputs["old"].(string)
+	new, _ := inputs["new"].(string)
+
+	// Default to replace all (-1)
+	count := -1
+	if n, ok := inputs["count"].(int); ok {
+		count = n
+	}
+
+	result := strings.Replace(str, old, new, count)
+	return map[string]interface{}{"result": result}, nil
+}