Add Slack channel

[tuchg]

Goal

Add Slack as a Lucarne channel adapter.

Scope

• Map Lucarne workspaces to Slack channel/thread primitives.
• Implement inbound messages, interactivity, file download/upload, and outbound formatting.
• Add configuration, onboarding/docs, and tests.

Acceptance criteria

• Users can run Lucarne through Slack with workspace-style conversations.
• Agent notifications, replies, approvals, and status commands work.
• OAuth/token setup is documented.
• Channel-specific behavior stays in the Slack adapter.