Slack OAuth callback

This Worker is alive. Configure Slack apps with one of these redirect URL patterns:

https://slack.edenspiekercommand.com/slack/oauth/callback
https://slack.edenspiekercommand.com/slack/oauth/callback/second-agent

The default app uses SLACK_CLIENT_ID / SLACK_CLIENT_SECRET. Named apps use suffixed secrets, e.g. SLACK_CLIENT_ID_SECOND_AGENT / SLACK_CLIENT_SECRET_SECOND_AGENT.