fix: remove shadowed os import in cluster_info

Inline `import os` at line 663 shadowed the top-level import,
causing flake8 F402.

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

stack_orchestrator/deploy/k8s/cluster_info.py

@@ -660,8 +660,6 @@ class ClusterInfo:
             )
 
             # Derive job name from file path: docker-compose-<name>.yml -> <name>
-            import os
-
             base = os.path.basename(job_file)
             # Strip docker-compose- prefix and .yml suffix
             job_name = base