[Feature Request] Hide /install-slack-app from the command menu once the Slack app is already installed

Once you've connected the Claude Slack app, /install-slack-app keeps showing up in the / command menu like it's still waiting to be installed. It isn't anything you can act on at that point, it's just clutter in a menu that should only surface relevant commands.

Repro:

1. Install and connect the Claude Slack app.
2. Open any Claude Code session and type / to bring up the command menu.
3. /install-slack-app still appears.

Request: When Claude Code detects the Slack app is already installed/connected, suppress /install-slack-app from the slash menu (or tuck it into a "manage" or "settings" section). Same logic would apply to any other "install X" command once X is actually installed.

Minor/cosmetic, not urgent, but it does make the menu feel a little off.