Add S-expression recipe support

- Add format detection that correctly handles ; comments
- Import artdag.sexp parser/compiler with YAML fallback
- Add execute_step_sexp and run_plan_sexp Celery tasks
- Update recipe upload to handle both S-expr and YAML formats

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

celery_app.py

@@ -14,7 +14,7 @@ app = Celery(
     'art_celery',
     broker=REDIS_URL,
     backend=REDIS_URL,
-    include=['legacy_tasks', 'tasks', 'tasks.analyze', 'tasks.execute', 'tasks.orchestrate']
+    include=['legacy_tasks', 'tasks', 'tasks.analyze', 'tasks.execute', 'tasks.orchestrate', 'tasks.execute_sexp']
 )
 
 app.conf.update(