all: support hiding subconvo output

Some of it is systematically noisy.
diff --git a/termui/termui.go b/termui/termui.go
index 1ffe655..7e4def9 100644
--- a/termui/termui.go
+++ b/termui/termui.go
@@ -150,6 +150,9 @@
 		if resp == nil {
 			return
 		}
+		if resp.HideOutput {
+			continue
+		}
 		// Typically a user message will start the thinking and a (top-level
 		// conversation) end of turn will stop it.
 		thinking := !(resp.EndOfTurn && resp.ParentConversationID == nil)