Add Slack Channel

This skill adds Slack support to NanoClaw, then walks through interactive setup.

Phase 1: Pre-flight

Check if already applied

Check if src/channels/slack.ts exists. If it does, skip to Phase 3 (Setup). The code changes are already in place.

Ask the user

Do they already have a Slack app configured? If yes, collect the Bot Token and App Token now. If no, we'll create one in Phase 3.

Phase 2: Apply Code Changes

Ensure channel remote