disk_usage_check.sh built its mount list with `df`, which blocks on a wedged
filesystem (stale NFS export, a stuck ZFS pool) — and that stalled the whole
check past the scheduler's 30s budget, leaving host:hubris:4 perpetually down.
Build the mount list from /proc/mounts (a read that never stats anything), and
bound every per-mount `df` with `timeout 8` so a single stuck mount is skipped
instead of hanging the probe. Degrades to plain `df` on hosts without
`timeout`//proc/mounts (macOS), whose local mounts don't hang.
dtoro
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