fix: Slack gateway setup missing event subscriptions and scopes

The 'hermes gateway setup' instructions for Slack were missing:
- The 'Subscribe to Events' step entirely (message.im, message.channels,
  app_mention, message.groups)
- Several required scopes (app_mentions:read, groups:history, users:read,
  files:write)
- Warning about bot only working in DMs without message.channels
- Step to invite the bot to channels

The 'hermes setup' flow (setup.py) and the website docs (slack.md)
already had the correct information — only gateway.py was outdated.

Reported by JordanB on Slack.

hermes_cli/gateway.py

@@ -482,14 +482,19 @@ _PLATFORMS = [
         "token_var": "SLACK_BOT_TOKEN",
         "setup_instructions": [
             "1. Go to https://api.slack.com/apps → Create New App → From Scratch",
-            "2. Enable Socket Mode: App Settings → Socket Mode → Enable",
-            "3. Get Bot Token: OAuth & Permissions → Install to Workspace → copy xoxb-... token",
-            "4. Get App Token: Basic Information → App-Level Tokens → Generate",
-            "   Name it anything, add scope: connections:write → copy xapp-... token",
-            "5. Add bot scopes: OAuth & Permissions → Scopes → chat:write, im:history,",
-            "   im:read, im:write, channels:history, channels:read",
-            "6. Reinstall the app to your workspace after adding scopes",
+            "2. Enable Socket Mode: Settings → Socket Mode → Enable",
+            "   Create an App-Level Token with scope: connections:write → copy xapp-... token",
+            "3. Add Bot Token Scopes: Features → OAuth & Permissions → Scopes",
+            "   Required: chat:write, app_mentions:read, channels:history, channels:read,",
+            "   groups:history, im:history, im:read, im:write, users:read, files:write",
+            "4. Subscribe to Events: Features → Event Subscriptions → Enable",
+            "   Required events: message.im, message.channels, app_mention",
+            "   Optional: message.groups (for private channels)",
+            "   ⚠ Without message.channels the bot will ONLY work in DMs!",
+            "5. Install to Workspace: Settings → Install App → copy xoxb-... token",
+            "6. Reinstall the app after any scope or event changes",
             "7. Find your user ID: click your profile → three dots → Copy member ID",
+            "8. Invite the bot to channels: /invite @YourBot",
         ],
         "vars": [
             {"name": "SLACK_BOT_TOKEN", "prompt": "Bot Token (xoxb-...)", "password": True,