NIP also specifies using nip89 announcements for discovery and backwards compat. Agent Cards should be included in nip89 announcements, I'll add that requirement
NIP89 was made for specific thing - declaring which url to use as template and how to put npub/nevent/etc to the app's url to open events in an app. IMO it's use for DVMs was a mistake, and I suggest you create a new kind for announcing agents with all the necessary metadata for this specific use case.