feat(workflow): implement a workflow node for knowledge base retrieval * feature/20251219_myh: (3 commits) feat(workflow): support multi-variable assignment in assigner node feat(workflow): implement a workflow node for knowledge base retrieval fix(template): remove default initial model in templates Signed-off-by: Eternity <1533512157@qq.com> Reviewed-by: zhuwenhui5566@163.com <zhuwenhui5566@163.com> Merged-by: zhuwenhui5566@163.com <zhuwenhui5566@163.com> CR-link: https://codeup.aliyun.com/redbearai/python/redbear-mem-open/change/41