fix(setup): add Interactivity & Shortcuts step to Slack setup

Slack interactive buttons (channel approval cards) require Interactivity
to be enabled in the app settings. Without it, button clicks silently
fail to reach the host. Added the step to both the setup wizard
post-install checklist and the add-slack SKILL.md.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.claude/skills/add-slack/SKILL.md

@@ -76,7 +76,13 @@ pnpm run build
 10. Under **Subscribe to bot events**, add:
     - `message.channels`, `message.groups`, `message.im`, `app_mention`
 11. Click **Save Changes**
-12. Slack will show a banner asking you to **reinstall the app** — click it to apply the new event subscriptions
+
+### Interactivity
+
+12. Go to **Interactivity & Shortcuts** and toggle **Interactivity** on
+13. Set the **Request URL** to the same `https://your-domain/webhook/slack`
+14. Click **Save Changes**
+15. Slack will show a banner asking you to **reinstall the app** — click it to apply the new settings
 
 ### Configure environment