subagent: keep runs at +1000 when their cgroup is unavailable

Without a run cgroup (no delegation, setup failed, or one create
failed) a subagent run was started plain at oom_score_adj 0, below the
+500 the unit gave it before. Every run of the daemon now goes through
the entry wrapper; it sets +1000 and enters the cgroup only when there
is one. run_cgroup::entry takes the cgroup as an Option.

hive-subagent-mcp/src/main.rs

@@ -65,7 +65,7 @@ async fn serve() -> Result<()> {
             .on_runtime(runtime)
             .publishing_to(hive_subagent_mcp::swarm_term::Publisher::spawn())
             .reading_models_from(hive_subagent_mcp::swarm_models::Reader::spawn())
-            .in_run_cgroups(hive_agent_sock::run_cgroup::RunCgroups::init("runs")),
+            .entering_runs(hive_agent_sock::run_cgroup::RunCgroups::init("runs")),
     );
 
     // Serve the MCP tools over streamable-http forever. No background poll