Add Slack/Discord chat platform adapter

[regnull]

Goal

Add a Slack and/or Discord adapter so OpenBot threads can be driven from those platforms (roadmap: chat platform adapters).

Context

OpenBot already has actors, inboxes, webhooks, and a Telegram settings surface. Slack/Discord should map external messages into threads/actors without reinventing the runtime.

Acceptance criteria

• Choose one platform for the first PR (Slack or Discord); note the other as follow-up
• Inbound: platform message → OpenBot thread message (correct actor / bot addressing)
• Outbound: bot replies (and ideally questions/approvals) visible on the platform
• Config/docs for tokens, webhooks/gateway, and security caveats
• Basic tests or a recorded manual demo walkthrough

Labels

help-wanted, enhancement