With reply_in_thread: false the whole channel is one session and the bot
answers top-level, so an unmentioned top-level message there is a
follow-up in a conversation the bot is part of, like a thread reply.
reply_expected is now False for a free-channel message only when it starts
its own session (a new top-level thread), else None. The bot-id set is
built inside _slack_reply_expected, as _channel_gate_allows does.
b17e037e35