Fix false positives in variable dependency validation

Only parse {variable} patterns in PromptSteps, not CodeSteps.

In CodeSteps, variables are available directly as Python variables
(e.g., use `input` not `{input}`). The {var} syntax in code is
typically Python f-string or .format() syntax, not CmdForge
template substitution.

This fixes false positives like artifact-ai where Python f-strings
like f"{format_guide}" were incorrectly flagged as missing CmdForge
variable references.

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

src/cmdforge/gui/pages/tool_builder_page.py

@@ -558,7 +558,10 @@ class ToolBuilderPage(QWidget):
     def _get_step_variable_refs(self, step) -> set:
         """Extract variable references from a step.
 
-        Parses {variable} patterns from prompt templates and code.
+        Parses {variable} patterns from prompt templates.
+        Note: CodeSteps are NOT parsed because variables are available directly
+        as Python variables, and {var} in code is typically Python f-string
+        or .format() syntax, not CmdForge substitution.
         """
         import re
         refs = set()
@@ -567,15 +570,11 @@ class ToolBuilderPage(QWidget):
         pattern = r'\{(\w+)\}'
 
         if isinstance(step, PromptStep):
-            # Parse prompt template
+            # Parse prompt template - these ARE CmdForge substitutions
             matches = re.findall(pattern, step.prompt or "")
             refs.update(matches)
-        elif isinstance(step, CodeStep):
-            # Parse code for variable references
-            # In code, variables are accessed directly, but we also check for {var} patterns
-            # since the code might use string formatting
-            matches = re.findall(pattern, step.code or "")
-            refs.update(matches)
+        # CodeStep: Don't parse - variables are available as Python vars directly
+        # and {var} syntax is typically Python string formatting
 
         return refs