Extend the gateway_restart_notification flag to cover
_notify_active_sessions_of_shutdown — the message that fires just
before drain ("⚠️ Gateway restarting — Your current task will be
interrupted. Send any message after restart and I'll try to resume
where you left off.") sent to active sessions and home channels.
Same operator/end-user reasoning: on a Slack workspace shared with
end users, "Gateway restarting" reads as "the bot is broken" — the
operator should be able to suppress it consistently with the other
two lifecycle pings rather than having a partial opt-out.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>