chatwait: return immediately when session is idle

Fixes a race where the client starts a new chatwait read after the
turn has ended (EnsureTrailingNewline already fired), causing a
5-second blocking delay before the shell prompt reappears.

store/sessionfile.go

@@ -198,6 +198,9 @@ func (h *sessionHelper) fileSpec(name string, mode os.FileMode) FileSpec {
 			ctx, cancel := context.WithCancel(connCtx)
 			defer cancel()
 			context.AfterFunc(h.sess.Ctx, cancel)
+			if h.sess.Core.State() == "idle" {
+				return nil, nil
+			}
 			offset := 0
 			if base == "" {
 				n, _ := h.sess.LogInfo()