feat: Propagate workflow plugins to Go, Rust, C++, and Mojo

Multi-language workflow plugin system following Python's structure:
- Each plugin in its own directory with implementation + package.json
- Category-level package.json manifests listing all plugins
- Consistent interface: run(runtime, inputs) -> outputs

Languages added:
- Go: math, string, logic, list, dict, var, convert (25+ plugins)
- Rust: math, string, logic, list, convert, var (50+ functions)
- C++: header-only math, string, logic, var, convert (30+ plugins)
- Mojo: math, string, list with systems programming features

Python structure fixed:
- Reorganized flat files into plugin subdirectories
- Added package.json metadata to all 120+ plugins
- Added missing backend category (15 plugins)
- Category manifests with plugin lists

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

workflow/plugins/go/string/string_replace.go

@@ -0,0 +1,28 @@
+// Package string provides string manipulation workflow plugins.
+package string
+
+import (
+	"strings"
+
+	plugin "metabuilder/workflow/plugins/go"
+)
+
+// Replace replaces occurrences in a string.
+func Replace(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
+}