Rule could-be-keyword-tags: Ignore some builtin tags

Rule could-be-keyword-tags: Ignore some builtin tags: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). In my code base, it keeps raising situations where all keywords use robot:flatten or robot:private . These tags are instructions for the Robot Framework runtime…