Add Slack channel support

[owainlewis]

Goal

Support Slack as a work-oriented messaging channel after the core gateway is production-ready.

Why

Slack is valuable for workspaces, but brings workspace auth and permission complexity that should come after Telegram.

Scope

• Configure Slack app credentials and allowed users/channels.
• Receive app mentions or DMs.
• Send replies to the correct thread or DM.
• Preserve channel-specific thread identity for routing and state.

Acceptance criteria

• Allowlisted Slack messages can route to Claude Code or Codex.
• Replies land in the expected Slack conversation or thread.
• The setup docs explain required Slack scopes.
• Slack state does not collide with other channels.