fix: soft-fail code detection in prompt pipeline

Change from sys.exit(1) to logging.warning when Python code is
detected in prompt.txt. Let test_syntax.py in the grader handle
scoring (2 pts deduction) rather than hard-failing the pipeline,
which would prevent all grading feedback.

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

Author: beachdweller

prompt_pipeline/entrypoint.py

@@ -85,11 +85,11 @@ def main() -> None:
         sys.exit(1)
 
     if contains_python_code(student_prompt):
-        logging.error(
-            "Prompt contains Python code constructs. "
+        logging.warning(
+            "Prompt appears to contain Python code constructs. "
+            "This will be penalized by the grader. "
             "Write a natural language description, not code."
         )
-        sys.exit(1)
 
     logging.info("Prompt length: %d chars", len(student_prompt))