Members set a personal greet with NickServ SET GREET; a channel founder
enables display with BotServ SET <#channel> GREET ON. On join, if greets
are enabled and the member has channel access and a non-empty greet, the
assigned bot shows [account] greet in the channel.
Per-account greet is a typed field on Account (AccountGreetSet event, rides
the account snapshot); the per-channel toggle is a new ChanSetting::BotGreet
bool. Greet is public in NickServ INFO.