feat: make notifyBotToken/adminUserId optional

The client never sends pairing notifications (the server does); these
Discord fields were required but unused. Make them optional + drop from
the config schema's required list. Back-compat: still accepted if set.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

plugin/openclaw.plugin.json

@@ -14,6 +14,6 @@
       "notifyBotToken": { "type": "string" },
       "adminUserId": { "type": "string" }
     },
-    "required": ["mainHost", "identifier", "notifyBotToken", "adminUserId"]
+    "required": ["mainHost", "identifier"]
   }
 }