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_length/string_length.mojo

@@ -1,35 +1,52 @@
-# Workflow plugin: get string length
-#
-# Returns the length of a string.
-# Input: {"text": "hello"}
-# Output: {"result": 5}
+"""Workflow plugin: get string length.
+
+Returns the length of a string.
+Input: {"text": "hello"}
+Output: {"result": 5}
+"""
 
 from collections import Dict
 from python import PythonObject
 
 
-fn run(inputs: Dict[String, PythonObject]) -> Dict[String, PythonObject]:
-    """Get the length of a string.
+struct StringLength:
+    """Plugin that gets the length of a string."""
 
-    Args:
-        inputs: Dictionary containing "text" key with the string to measure.
+    var node_type: String
+    var category: String
+    var description: String
 
-    Returns:
-        Dictionary with "result" key containing the string length as integer.
-    """
-    var text = String(inputs.get("text", PythonObject("")))
-    var output = Dict[String, PythonObject]()
+    fn __init__(inout self):
+        """Initialize the StringLength plugin."""
+        self.node_type = "string.length"
+        self.category = "string"
+        self.description = "Get the length of a string"
 
-    var length = len(text)
+    fn execute(self, inputs: Dict[String, PythonObject], runtime: PythonObject = PythonObject(None)) -> Dict[String, PythonObject]:
+        """Get the length of a string.
 
-    output["result"] = PythonObject(length)
-    return output
+        Args:
+            inputs: Dictionary containing "text" key with the string to measure.
+            runtime: Optional runtime context (unused).
+
+        Returns:
+            Dictionary with "result" key containing the string length as integer.
+        """
+        var text = String(inputs.get("text", PythonObject("")))
+        var output = Dict[String, PythonObject]()
+
+        var length = len(text)
+
+        output["result"] = PythonObject(length)
+        return output
 
 
 fn main():
     """Test the length plugin."""
+    var plugin = StringLength()
+
     var inputs = Dict[String, PythonObject]()
     inputs["text"] = PythonObject("hello")
 
-    var result = run(inputs)
+    var result = plugin.execute(inputs)
     print("Length:", result["result"])  # Expected: 5