Package disk reports free space on a runner host's image store. It stats the
filesystem rather than summing file sizes: VM disk images are sparse, so what
they appear to hold bears no relation to the blocks they occupy.
Usage is a filesystem's available and total capacity in bytes. Free counts
only blocks available to an unprivileged writer — the reserve root can still
dip into can't run a job.
Stat reports usage for the filesystem holding path. A path that doesn't exist
yet — an image store before the first pull — is measured at its nearest
existing ancestor.