container stats: measure agent disk usage through hive-priv, unknown on failure

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

hive-c0re/src/coordinator.rs

@@ -1130,14 +1130,6 @@ impl Coordinator {
         crate::paths::agent_state_dir(name).join("state")
     }
 
-    /// Per-agent working dir for worktrees and build dirs. Bind-mounted RW
-    /// into the agent container at `/agents/{name}/work`. Survives
-    /// destroy/recreate. A sibling of `state/` rather than inside it, so a
-    /// claude started under `work/` never walks up into `state/CLAUDE.md`.
-    pub fn agent_work_dir(name: &hive_types::Ident) -> PathBuf {
-        crate::paths::agent_state_dir(name).join(crate::paths::WORK_SUBDIR)
-    }
-
     /// Per-agent harness-internal state dir. Bind-mounted RW into the
     /// agent container at `/agents/{name}/harness`. Holds sqlite dbs
     /// and config files owned by the harness (`hyperhive-events.sqlite`,