The disk sampler ran `du -sxb` as the unprivileged hive-core user over
each agent's state/, work/ and container rootfs. hive-core cannot read
the rootfs or all of the agent-owned files, so `du` failed and each
failed path silently counted as 0: the dashboard showed a few MB for
agents using over 13 GB.
Add a hive-priv `MeasureAgentDisk { agent_name }` op. hive-priv
validates the name, derives the three paths itself (the request carries
no path), skips the ones that don't exist, and runs one
`du -s -x -b -c -P` as root with a 10 min timeout. A timeout, a non-zero
exit or output without a `total` line is an error.
hive-c0re calls it every 30 min instead of running `du` every 5 min:
each pass walks every agent's full tree as root on the disk the agents
work on, and the sizes move slowly. A failed measurement is cached as
None, so `disk_bytes` is null, the OTel storage.usage sample is omitted
and the dashboard shows "unknown", never 0.
`Coordinator::agent_work_dir` had no other caller and is removed.
Closes#4956
98ccc31e2b