fix: remove {{ }} from placeholder keys before resolution

Placeholder resolver returns keys with {{ }} wrappers,
but resolve_placeholders expects clean keys.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/prompt_executor.py

@@ -380,7 +380,12 @@ async def execute_prompt_with_data(
     # This makes old-style prompts work with the new executor
     try:
         processed_placeholders = get_placeholder_example_values(profile_id)
-        variables.update(processed_placeholders)
+        # Remove {{ }} from keys (placeholder_resolver returns them with wrappers)
+        cleaned_placeholders = {
+            key.replace('{{', '').replace('}}', ''): value
+            for key, value in processed_placeholders.items()
+        }
+        variables.update(cleaned_placeholders)
     except Exception as e:
         # Continue even if placeholder resolution fails
         if enable_debug: