Add Python syntax checking to Code Step dialog

Validates code with ast.parse() before accepting. Shows line number
and error message if syntax is invalid.

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

src/cmdforge/gui/dialogs/step_dialog.py

@@ -1,9 +1,11 @@
 """Step editor dialogs."""
 
+import ast
+
 from PySide6.QtWidgets import (
     QDialog, QVBoxLayout, QFormLayout, QLineEdit,
     QComboBox, QPushButton, QHBoxLayout, QLabel,
-    QPlainTextEdit, QSplitter, QGroupBox, QTextEdit
+    QPlainTextEdit, QSplitter, QGroupBox, QTextEdit, QMessageBox
 )
 from PySide6.QtCore import Qt, QThread, Signal
 
@@ -378,6 +380,18 @@ IMPORTANT: Return ONLY executable inline Python code. No function definitions, n
             self.output_input.setFocus()
             return
 
+        # Syntax check the Python code
+        try:
+            ast.parse(code)
+        except SyntaxError as e:
+            line_info = f" (line {e.lineno})" if e.lineno else ""
+            QMessageBox.warning(
+                self, "Syntax Error",
+                f"Python syntax error{line_info}:\n\n{e.msg}"
+            )
+            self.code_input.setFocus()
+            return
+
         self.accept()
 
     def get_step(self) -> CodeStep: