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>

setup/channels/slack.ts

@@ -381,7 +381,15 @@ function showPostInstallChecklist(info: WorkspaceInfo): void {
         `     • Request URL: https://<your-public-host>/webhook/slack`,
         '     • Subscribe to bot events: message.channels, message.groups,',
         '       message.im, app_mention',
-        '     • Save, then reinstall the app when Slack prompts',
+        '     • Save Changes',
+        '',
+        '  3. In your Slack app → Interactivity & Shortcuts:',
+        '     • Toggle "Interactivity" on',
+        `     • Request URL: https://<your-public-host>/webhook/slack`,
+        '     • Save Changes',
+        '',
+        '  4. Slack will prompt you to reinstall the app — do it to apply',
+        '     the new settings',
       ].join('\n'),
       6,
     ),