Adds Discord gateway: REST API POST /channels/{id}/messages for
sending, HTTP GET for polling messages. Maps Discord mentions to
:user-input signals. HITL commands intercepted before injection.
Adds Slack gateway: Web API chat.postMessage for sending,
conversations.history for polling. Uses SLACK_TOKEN from vault.
Each gateway registered in *gateway-registry* following the same
jail-loaded skill pattern as Telegram and Signal.
Registry now has 4 platforms: telegram, signal, discord, slack.
Amr Gharbeia
c7e9893e68