Default Discord client allowedMentions to users-only (block mass pings)

The raw sendMessage/editMessage paths did not route through
sanitizeForOutbound, so an agent-authored @everyone/@here in a progress
or edited message could still ping the whole channel. Set a client-level
allowedMentions default (parse: ['users']) so no normal send/edit can
mass-ping; explicit <@id> user mentions still work, and the disk-usage
alert broadcasts via a separate REST path with its own allowedMentions.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/channels/discord.ts

@@ -210,6 +210,11 @@ export class DiscordChannel implements Channel {
         GatewayIntentBits.MessageContent,
         GatewayIntentBits.DirectMessages,
       ],
+      // Guardrail: agent-authored messages must never mass-ping. Allow only
+      // explicit user mentions (e.g. <@id>) by default; never parse @everyone,
+      // @here, or role mentions on any send/edit. Intentional broadcasts (e.g.
+      // the disk-usage alert) use a dedicated path with their own allowedMentions.
+      allowedMentions: { parse: ['users'] },
     });
 
     this.client.on(Events.MessageCreate, async (message: Message) => {