Let task agents create and manage subtasks

internal/server/server.go

@@ -396,7 +396,25 @@ func (s *Server) MergeTask(w http.ResponseWriter, r *http.Request, taskID string
 		return
 	}
 
-	t, err = s.agents.Merge(r.Context(), t, p, u, req.MergeTitle)
+	if req.Target != nil && *req.Target == api.MergeTargetParent {
+		t, err = s.agents.MergeIntoParent(r.Context(), t, u)
+		if err != nil {
+			writeAgentError(w, err)
+			return
+		}
+		writeJSON(w, http.StatusAccepted, t)
+		return
+	}
+	if req.Target != nil && !req.Target.Valid() {
+		writeError(w, http.StatusBadRequest, fmt.Errorf("unknown target %q", *req.Target))
+		return
+	}
+	if req.MergeTitle == nil {
+		writeError(w, http.StatusBadRequest, errors.New("merge_title is required to merge into the default branch"))
+		return
+	}
+
+	t, err = s.agents.Merge(r.Context(), t, p, u, *req.MergeTitle)
 	if err != nil {
 		writeAgentError(w, err)
 		return