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/python/string/string_replace/string_replace.py

@@ -0,0 +1,12 @@
+"""Workflow plugin: replace in string."""
+
+
+def run(_runtime, inputs):
+    """Replace occurrences in string."""
+    text = inputs.get("text", "")
+    old = inputs.get("old", "")
+    new = inputs.get("new", "")
+    count = inputs.get("count", -1)
+
+    result = text.replace(old, new, count)
+    return {"result": result}