refactor(workflow): convert all plugins to class/struct + factory pattern

- Python: class extending NodeExecutor + factory.py (80+ plugins)
- TypeScript: class implements NodeExecutor + factory.ts (7 groups, 116 classes)
- Go: struct with methods + factory.go (36 plugins)
- Rust: struct impl NodeExecutor trait + factory.rs (54 plugins)
- Mojo: struct + factory.mojo (11 plugins)

All package.json files now include:
- files array listing source files
- metadata.class/struct field
- metadata.entrypoint field

This enables a unified plugin loading system across all languages
with no import side effects (Spring-style DI pattern).

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

workflow/plugins/mojo/string/string_lower/string_lower.mojo

@@ -1,37 +1,54 @@
-# Workflow plugin: convert string to lowercase
-#
-# Converts a string to all lowercase characters.
-# Input: {"text": "HELLO WORLD"}
-# Output: {"result": "hello world"}
+"""Workflow plugin: convert string to lowercase.
+
+Converts a string to all lowercase 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 lowercase.
+struct StringLower:
+    """Plugin that converts a string to lowercase."""
 
-    Args:
-        inputs: Dictionary containing "text" key with the string to convert.
+    var node_type: String
+    var category: String
+    var description: String
 
-    Returns:
-        Dictionary with "result" key containing the lowercase string.
-    """
-    var text = String(inputs.get("text", PythonObject("")))
-    var output = Dict[String, PythonObject]()
+    fn __init__(inout self):
+        """Initialize the StringLower plugin."""
+        self.node_type = "string.lower"
+        self.category = "string"
+        self.description = "Convert a string to lowercase"
 
-    # Convert to lowercase using Python interop for full Unicode support
-    var py_text = PythonObject(text)
-    var lower_text = py_text.lower()
+    fn execute(self, inputs: Dict[String, PythonObject], runtime: PythonObject = PythonObject(None)) -> Dict[String, PythonObject]:
+        """Convert a string to lowercase.
 
-    output["result"] = lower_text
-    return output
+        Args:
+            inputs: Dictionary containing "text" key with the string to convert.
+            runtime: Optional runtime context (unused).
+
+        Returns:
+            Dictionary with "result" key containing the lowercase string.
+        """
+        var text = String(inputs.get("text", PythonObject("")))
+        var output = Dict[String, PythonObject]()
+
+        # Convert to lowercase using Python interop for full Unicode support
+        var py_text = PythonObject(text)
+        var lower_text = py_text.lower()
+
+        output["result"] = lower_text
+        return output
 
 
 fn main():
     """Test the lower plugin."""
+    var plugin = StringLower()
+
     var inputs = Dict[String, PythonObject]()
     inputs["text"] = PythonObject("HELLO WORLD")
 
-    var result = run(inputs)
+    var result = plugin.execute(inputs)
     print("Lowercase:", result["result"])  # Expected: "hello world"