feat(elder): @Elder merges a thread's delivers when asked (#26)

nestlog.go

@@ -158,6 +158,25 @@ func (s *MessageStore) NudgeFor(nestID, targetMsgID string) (Message, bool) {
 	return Message{}, false
 }
 
+// ReportsTo lists messages that answer a given referent, oldest first.
+func (s *MessageStore) ReportsTo(nestID, referentID string, kinds ...string) []Message {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	var out []Message
+	for _, m := range s.log[nestID] {
+		if m.RefersTo != referentID {
+			continue
+		}
+		for _, k := range kinds {
+			if m.Kind == k {
+				out = append(out, m)
+				break
+			}
+		}
+	}
+	return out
+}
+
 func (s *MessageStore) save(nestID string) error {
 	if err := os.MkdirAll(s.dir, 0o755); err != nil {
 		return err