fix: bypass DM sessions without metadata and make tool globally visible

1. DM bypass: when neither senderId nor channelId can be extracted from
   the prompt (DM sessions lack untrusted conversation info), skip the
   no-reply gate and allow the message through with end-marker injection.

2. Tool visibility: change whispergateway_tools registration from
   optional=true to optional=false so all agents can see the tool
   without needing explicit tools.allow entries.

plugin/index.ts

@@ -355,7 +355,7 @@ export default {
           return { content: [{ type: "text", text: `unsupported action: ${action}` }], isError: true };
         },
       },
-      { optional: true },
+      { optional: false },
     );
 
     api.on("message_received", async (event, ctx) => {