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

nix/agent-modules/user.nix

@@ -195,7 +195,7 @@ in
           : > "$marker"
         fi
       fi
-      # Scope state + harness chowns to THIS container's own dirs only.
+      # Scope state + harness + work chowns to THIS container's own dirs only.
       # The glob `/agents/*/state` also matches other agents' state dirs
       # bind-mounted into a `ManageRootAgent` holder's container, which
       # would clobber the ownership those dirs' own activation scripts set
@@ -209,6 +209,9 @@ in
       if [ -d "/agents/$userName/harness" ]; then
         chown -hR "$userName:$userName" "/agents/$userName/harness" 2>/dev/null || true
       fi
+      if [ -d "/agents/$userName/work" ]; then
+        chown -hR "$userName:$userName" "/agents/$userName/work" 2>/dev/null || true
+      fi
       # The socket dir is bind-mounted from the host, where hive-priv creates
       # it `0751 root`. The harness binds its sockets here as this user; the
       # 0751 is what lets hive-c0re and nginx dial them without listing