Disk-full Linux build host: count every top-level dir with du -x

When a shared Linux host's root fs hits 99%, du on /home and /tmp alone left ~135 GB unaccounted. A per-top-level loop (for d in /*; du -x -s) found a large vendor tool tree under /software on the same partition. Run it under ionice -c3 nice -n19 with a timeout so it doesn't disturb timed jobs. Hosts that never reboot also never clear /tmp, so hundreds of run-output dirs pile up there. Meanwhile a second, nearly empty NVMe disk mounted at /scratch sat unused. Fix the root cause by sending large outputs to the spare disk, then archive-and-verify each dir (upload, compare checksums) before deleting.