fix: Add variable substitution to code steps

Code steps were executing with literal {variable} placeholders instead
of substituted values. Now substitute_variables() is called on the code
before execution, matching the behavior of prompt steps.

This fixes issues like {outputfile} being treated as a Python set literal
or written as a literal filename.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/smarttools/runner.py

@@ -67,12 +67,15 @@ def execute_code_step(step: CodeStep, variables: dict) -> tuple[dict, bool]:
     Returns:
         Tuple of (output_vars_dict, success)
     """
+    # Substitute variables in code (like {outputfile} -> actual value)
+    code = substitute_variables(step.code, variables)
+
     # Create execution environment with variables
     local_vars = dict(variables)
 
     try:
-        # Execute the code
-        exec(step.code, {"__builtins__": __builtins__}, local_vars)
+        # Execute the code with substituted variables
+        exec(code, {"__builtins__": __builtins__}, local_vars)
 
         # Support comma-separated output vars (e.g., "a, b, c")
         output_vars = [v.strip() for v in step.output_var.split(',')]