ant: fix subconvo url

not sure how/when this regressed...
diff --git a/ant/ant.go b/ant/ant.go
index a3eb79a..2ba714e 100644
--- a/ant/ant.go
+++ b/ant/ant.go
@@ -440,6 +440,7 @@
 	return &Convo{
 		Ctx:           skribe.ContextWithAttr(c.Ctx, slog.String("convo_id", id), slog.String("parent_convo_id", c.ID)),
 		HTTPC:         c.HTTPC,
+		URL:           c.URL,
 		APIKey:        c.APIKey,
 		Model:         c.Model,
 		MaxTokens:     c.MaxTokens,