review: a cancelled turn keeps its trace, and a quiet box still expires (V-629)

Two defects found reviewing the PR.

The insert ran on the turn's own context, so a caller that hung up or timed out
cancelled it. That is exactly the turn worth having. It now runs detached, with
a one-second bound of its own, because a write must not hold the reply.

Retention was enforced on write alone, so a box that goes quiet for a month kept
every row until the next sixty-fourth turn. pruneTracesOnStart closes that, and
RoutingTraceRetention is exported so the daemon reads the same number the store
enforces.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0117tgnmbgZpHVV3XSNw8Qua

internal/store/routingtraces_test.go

@@ -62,7 +62,7 @@ func TestPruneRoutingTracesByAge(t *testing.T) {
 			t.Fatal(err)
 		}
 	}
-	if err := s.PruneRoutingTraces(ctx, now.Add(-routingTraceRetention)); err != nil {
+	if err := s.PruneRoutingTraces(ctx, now.Add(-RoutingTraceRetention)); err != nil {
 		t.Fatal(err)
 	}
 	got, err := s.RecentRoutingTraces(ctx, 10)