Restore AI-assisted code generation in Code Step dialog

Feature was present in TUI but missing after GUI conversion. Now includes:
- Split layout: Code editor (left) | AI Assist panel (right)
- Provider selector dropdown for AI calls
- Prompt editor with smart default template showing available variables
- "Generate Code" button that calls AI in background thread
- Response feedback area showing success/error status
- Automatic markdown fence stripping from AI responses
- Available variables computed from tool arguments + previous step outputs

Co-Authored-By: Claude Opus 4.5 <[email protected]>

src/cmdforge/gui/pages/tool_builder_page.py

@@ -261,6 +261,34 @@ class ToolBuilderPage(QWidget):
         del self._tool.arguments[idx]
         self._refresh_arguments()
 
+    def _get_available_vars(self, up_to_step: int = -1) -> list:
+        """Get available variables for a step.
+
+        Args:
+            up_to_step: Include variables from steps before this index.
+                       -1 means include all existing steps (for new steps).
+        """
+        vars_list = ["input"]
+
+        # Add argument variables
+        if self._tool and self._tool.arguments:
+            for arg in self._tool.arguments:
+                if arg.variable and arg.variable not in vars_list:
+                    vars_list.append(arg.variable)
+
+        # Add output variables from previous steps
+        if self._tool and self._tool.steps:
+            steps_to_check = self._tool.steps if up_to_step < 0 else self._tool.steps[:up_to_step]
+            for step in steps_to_check:
+                if hasattr(step, 'output_var') and step.output_var:
+                    # Handle comma-separated output vars
+                    for var in step.output_var.split(','):
+                        var = var.strip()
+                        if var and var not in vars_list:
+                            vars_list.append(var)
+
+        return vars_list
+
     def _add_prompt_step(self):
         """Add a prompt step."""
         from ..dialogs.step_dialog import PromptStepDialog
@@ -275,7 +303,8 @@ class ToolBuilderPage(QWidget):
     def _add_code_step(self):
         """Add a code step."""
         from ..dialogs.step_dialog import CodeStepDialog
-        dialog = CodeStepDialog(self)
+        available_vars = self._get_available_vars()
+        dialog = CodeStepDialog(self, available_vars=available_vars)
         if dialog.exec():
             step = dialog.get_step()
             if not self._tool:
@@ -297,7 +326,8 @@ class ToolBuilderPage(QWidget):
             dialog = PromptStepDialog(self, step)
         elif isinstance(step, CodeStep):
             from ..dialogs.step_dialog import CodeStepDialog
-            dialog = CodeStepDialog(self, step)
+            available_vars = self._get_available_vars(up_to_step=idx)
+            dialog = CodeStepDialog(self, step, available_vars=available_vars)
         else:
             return