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/rules.ts

@@ -73,6 +73,12 @@ export function evaluateDecision(params: {
     return { shouldUseNoReply: false, shouldInjectEndMarkerPrompt: false, reason: "non_discord" };
   }
 
+  // DM bypass: if no conversation info was found in the prompt (no senderId AND no channelId),
+  // this is a DM session where untrusted metadata is not injected. Always allow through.
+  if (!params.senderId && !params.channelId) {
+    return { shouldUseNoReply: false, shouldInjectEndMarkerPrompt: true, reason: "dm_no_metadata_bypass" };
+  }
+
   const policy = resolvePolicy(config, params.channelId, params.channelPolicies);
 
   const mode = policy.listMode;