fix(setup): wire Slack agent during setup like Discord/Telegram

Slack setup previously stopped after installing the adapter, leaving
users to manually discover /init-first-agent. When they DM'd the bot,
the channel-approval flow silently failed because no owner existed.

Now the Slack setup flow matches Discord/Telegram:
- Collects the operator's Slack member ID
- Opens a DM channel via conversations.open (requires im:write scope)
- Runs init-first-agent to establish ownership, wiring, and welcome DM
- Updates post-install note to focus on webhook URL (the only remaining step)

The welcome DM is delivered via chat.postMessage (outbound), which works
before Event Subscriptions are configured. The user sees the greeting
immediately; inbound replies require webhooks.

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

setup/auto.ts

@@ -510,10 +510,7 @@ function channelDmLabel(choice: ChannelChoice): string | null {
     case 'imessage':
       return 'iMessage';
     case 'slack':
-      // Slack install doesn't wire an agent or send a welcome DM — the
-      // driver prints its own "finish in your Slack app" note. Falling
-      // through to null avoids a misleading "check your Slack DMs" banner.
-      return null;
+      return 'Slack DMs';
     default:
       return null;
   }