docs(slack): add missing DM scopes to manifest (openclaw#29999) thanks @JcMinarro

Verified:
- pnpm install --frozen-lockfile
- pnpm build
- pnpm check
- pnpm test:macmini

Co-authored-by: JcMinarro <4047514+JcMinarro@users.noreply.github.com>
Co-authored-by: Tak Hoffman <781889+Takhoffman@users.noreply.github.com>

docs/channels/slack.md

@@ -359,7 +359,11 @@ Notes:
         "channels:read",
         "groups:history",
         "im:history",
+        "im:read",
+        "im:write",
         "mpim:history",
+        "mpim:read",
+        "mpim:write",
         "users:read",
         "app_mentions:read",
         "assistant:write",