fix: improve dry-run mode and error handling

- Export MOLE_DRY_RUN env var for subprocess visibility
- Add || true to grep commands to prevent pipeline failures
- Add dry-run test for clean_orphaned_system_services
- Simplify clean_local_snapshots tests

bin/clean.sh

@@ -1069,6 +1069,7 @@ main() {
                 ;;
             "--dry-run" | "-n")
                 DRY_RUN=true
+                export MOLE_DRY_RUN=1
                 ;;
             "--whitelist")
                 source "$SCRIPT_DIR/../lib/manage/whitelist.sh"