Posts made with a user token (xoxp-) arrive with app_id and no
client_msg_id, so _event_declares_bot_sender dropped them as app traffic;
the only workaround was allow_bots: all. Adds
platforms.slack.extra.api_human_users (SLACK_API_HUMAN_USERS fallback), a
users-only allowlist consulted inside the predicate.
Salvaged from #100964 (users only: an app-id allowlist would also admit
the app's own xoxb bot posts, which share the user+app_id shape).
8be47c19bf