Add $EDITOR button to open code in external editor

When editing a code step, users can now click the "$EDITOR" button
to open the current code in their preferred external editor:

- Uses $EDITOR environment variable (falls back to $VISUAL, then nano)
- Creates a temp .py file with current code
- Suspends urwid UI while editor runs
- Imports edited code back when editor closes
- Shows success/error status in the dialog

This allows using vim, nano, VS Code, or any editor for complex
code editing while still using the SmartTools UI for workflow.

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

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

src/smarttools/ui_urwid.py

@@ -1788,7 +1788,54 @@ Return ONLY the Python code, no explanations or markdown fencing."""
         def on_cancel(_):
             self.close_overlay()
 
+        def on_external_edit(_):
+            """Open code in external editor ($EDITOR)."""
+            import os
+            import subprocess
+            import tempfile
+
+            current_code = code_edit.edit_text
+
+            # Stop the urwid loop temporarily
+            if self.loop:
+                self.loop.stop()
+
+            try:
+                # Create temp file with current code
+                with tempfile.NamedTemporaryFile(mode='w', suffix='.py', delete=False) as f:
+                    f.write(current_code)
+                    temp_path = f.name
+
+                # Get editor from environment
+                editor = os.environ.get('EDITOR', os.environ.get('VISUAL', 'nano'))
+
+                # Run editor
+                subprocess.run([editor, temp_path], check=True)
+
+                # Read back the edited code
+                with open(temp_path, 'r') as f:
+                    new_code = f.read()
+
+                # Update the code editor
+                code_edit.set_edit_text(new_code)
+                status_text.set_text(('success', f"Code updated from {editor}"))
+
+                # Clean up temp file
+                os.unlink(temp_path)
+
+            except subprocess.CalledProcessError:
+                status_text.set_text(('error', "Editor exited with error"))
+            except FileNotFoundError:
+                status_text.set_text(('error', f"Editor '{editor}' not found"))
+            except Exception as e:
+                status_text.set_text(('error', f"Edit error: {e}"))
+            finally:
+                # Restart the urwid loop
+                if self.loop:
+                    self.loop.start()
+
         load_btn = Button3DCompact("Load", on_load)
+        edit_btn = Button3DCompact("$EDITOR", on_external_edit)
 
         # Code editor in a box - use ListBox for proper focus handling and scrolling
         # Wrap in DOSScrollBar for DOS-style scrollbar with arrow buttons
@@ -1814,6 +1861,8 @@ Return ONLY the Python code, no explanations or markdown fencing."""
                 ('weight', 1, urwid.AttrMap(file_edit, 'edit', 'edit_focus')),
                 ('pack', urwid.Text(" ")),
                 ('pack', load_btn),
+                ('pack', urwid.Text(" ")),
+                ('pack', edit_btn),
             ])),
             ('pack', status_text),
             ('weight', 1, main_columns),