test(events): prefer runtime hook observation

Use RuntimeEventObserver for the normal in-process hook observer path and make the process-hook helper assert hook.runtime_event notifications.

Validation: go test ./pkg/agent; make lint

pkg/agent/subturn_test.go

@@ -140,7 +140,7 @@ func TestSpawnSubTurn(t *testing.T) {
 				agent:          al.registry.GetDefaultAgent(),
 			}
 
-			// Subscribe to real EventBus to capture events
+			// Subscribe to runtime events to capture sub-turn lifecycle.
 			collector, collectCleanup := newEventCollector(t, al)
 			defer collectCleanup()