agent: make the OOM killer prefer a subagent over the agent's own turn

Both units ran at OOMScoreAdjust=0, so under container memory pressure the
kernel picked purely on footprint — and the agent's own claude is often
the fattest process in the container, which means the session supervising
the work died before the work did.

The sign is the load-bearing part and is easy to invert: a HIGHER
OOMScoreAdjust means MORE likely to be killed, because the kernel adds it
to the badness score it derives from the process's memory footprint and
then kills the highest scorer. So hive-subagent-daemon gets +500 (first in
line) and hive-agent gets -500 (last in line). Written backwards this
makes the reported bug worse rather than better, so module-eval pins the
order as an inequality.

Both values are inherited by the nested claude each unit spawns as a
child, so ordering the units orders the sessions underneath them. -500
rather than -1000 on the harness: fully exempting it would leave the
kernel nothing to kill in a container whose only large process is the
harness.

Refs #4316

nix/module-eval.nix

@@ -1385,6 +1385,23 @@ let
       name = "an agent with no byte-valued memory cap renders no subagent ceiling";
       ok = !((agentSubagentDaemon agentUncapped).serviceConfig ? MemoryHigh);
     }
+    {
+      # The sign is the whole property, and it is easy to write backwards:
+      # a HIGHER OOMScoreAdjust is a MORE likely victim. So the subagent
+      # daemon must be strictly above zero and the harness strictly below
+      # it — swap the two and the kernel takes the agent's own turn first,
+      # which is worse than setting nothing at all. Both nested `claude`
+      # processes inherit their unit's value, so ordering the units orders
+      # the sessions. Held as an inequality rather than two constants: what
+      # must not drift is the order, not the magnitudes.
+      name = "the OOM killer prefers a subagent over the agent's own session";
+      ok =
+        let
+          sub = (agentSubagentDaemon agentUncapped).serviceConfig.OOMScoreAdjust;
+          own = (agentHarness agentUncapped).serviceConfig.OOMScoreAdjust;
+        in
+        sub > 0 && own < 0 && sub > own;
+    }
     {
       # The doctrine three glue files state, as a property a rewrite has to
       # keep: a client is defined by holding a certificate the store accepts,