fix(cli): mon clean now properly removes containers and volumes

Summary

• Fix mon clean to properly remove containers and volumes using docker-compose down -v
• Clean up orphaned volumes from previous installs with different project names
• Add --keep-volumes option to preserve data if needed

The previous implementation only used docker volume prune which doesn't remove volumes attached to containers. This caused issues like Grafana password not working after reinstall (password stored in persisted grafana.db).

Test plan

• Run mon local-install, then mon clean, verify all volumes are removed
• Run mon local-install again, verify Grafana password works
• Test mon clean --keep-volumes preserves data

Relates to: https://gitlab.com/postgres-ai/platform/-/issues/346

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