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/mojo/string/string_upper/string_upper.mojo

@@ -0,0 +1,37 @@
+# Workflow plugin: convert string to uppercase
+#
+# Converts a string to all uppercase characters.
+# Input: {"text": "hello world"}
+# Output: {"result": "HELLO WORLD"}
+
+from collections import Dict
+from python import PythonObject
+
+
+fn run(inputs: Dict[String, PythonObject]) -> Dict[String, PythonObject]:
+    """Convert a string to uppercase.
+
+    Args:
+        inputs: Dictionary containing "text" key with the string to convert.
+
+    Returns:
+        Dictionary with "result" key containing the uppercase string.
+    """
+    var text = String(inputs.get("text", PythonObject("")))
+    var output = Dict[String, PythonObject]()
+
+    # Convert to uppercase using Python interop for full Unicode support
+    var py_text = PythonObject(text)
+    var upper_text = py_text.upper()
+
+    output["result"] = upper_text
+    return output
+
+
+fn main():
+    """Test the upper plugin."""
+    var inputs = Dict[String, PythonObject]()
+    inputs["text"] = PythonObject("hello world")
+
+    var result = run(inputs)
+    print("Uppercase:", result["result"])  # Expected: "HELLO WORLD"