docs: improve Slack setup guide with channel event subscriptions and scopes

The #1 support issue with Slack is 'bot works in DMs but not channels'.
This is almost always caused by missing event subscriptions (message.channels,
message.groups) or missing OAuth scopes (channels:history, groups:history).

Changes:
- slack.md: Move channels:history and groups:history from optional to required
  scopes. Move message.channels and message.groups to required events. Add new
  'How the Bot Responds' section explaining DM vs channel behavior. Add Step 8
  for inviting bot to channels. Expand troubleshooting table with specific
  'works in DMs not channels' entry. Add quick checklist for channel debugging.
- setup.py: Expand Slack setup wizard with all required scopes, event
  subscriptions, and a warning that without message.channels/message.groups
  the bot only works in DMs. Add link to full docs. Improve Member ID
  discovery instructions.
- config.py: Update SLACK_BOT_TOKEN and SLACK_APP_TOKEN descriptions to list
  required scopes and event subscriptions inline.

hermes_cli/config.py

@@ -401,14 +401,18 @@ OPTIONAL_ENV_VARS = {
         "category": "messaging",
     },
     "SLACK_BOT_TOKEN": {
-        "description": "Slack bot integration",
+        "description": "Slack bot token (xoxb-). Get from OAuth & Permissions after installing your app. "
+                       "Required scopes: chat:write, app_mentions:read, channels:history, groups:history, "
+                       "im:history, im:read, im:write, users:read, files:write",
         "prompt": "Slack Bot Token (xoxb-...)",
         "url": "https://api.slack.com/apps",
         "password": True,
         "category": "messaging",
     },
     "SLACK_APP_TOKEN": {
-        "description": "Slack Socket Mode connection",
+        "description": "Slack app-level token (xapp-) for Socket Mode. Get from Basic Information → "
+                       "App-Level Tokens. Also ensure Event Subscriptions include: message.im, "
+                       "message.channels, message.groups, app_mention",
         "prompt": "Slack App Token (xapp-...)",
         "url": "https://api.slack.com/apps",
         "password": True,