agents: persistent /agents/<n>/work dir outside state/

Agents keep git worktrees and cargo target dirs under /home/<agent>,
which lives in the container rootfs and is deleted on destroy. They
can't move them under state/, because a claude started there walks up
into state/CLAUDE.md and takes on the agent's identity.

Add a per-agent work/ dir next to state/ on the host
(/var/lib/hyperhive/agents/<n>/work), bind-mounted RW at
/agents/<n>/work. hive-c0re creates it in ensure_state_dir and in
set_nspawn_flags (the rebuild path reaches only the latter), the
agent's activation chowns it to the agent user like state/ and
harness/, and measure_agent_disk counts it. destroy never touches the
agent root, so work/ survives it; purge removes the whole root, so
work/ goes with it.

Closes #4977

hive-c0re/src/coordinator.rs

@@ -1130,6 +1130,14 @@ 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("work")
+    }
+
     /// 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`,