feat(discord): add silent message support (SUPPRESS_NOTIFICATIONS flag)

- Add silent option to message tool for Discord
- Passes SUPPRESS_NOTIFICATIONS flag (4096) to Discord API
- Threads silent param through entire outbound chain:
  - message-action-runner.ts
  - outbound-send-service.ts
  - message.ts
  - deliver.ts
  - discord outbound adapter
  - send.outbound.ts
  - send.shared.ts

Usage: message tool with silent=true suppresses push/desktop notifications

src/infra/outbound/message-action-runner.ts

@@ -820,6 +820,7 @@ async function handleSendAction(ctx: ResolvedActionContext): Promise<MessageActi
   params.message = message;
   const gifPlayback = readBooleanParam(params, "gifPlayback") ?? false;
   const bestEffort = readBooleanParam(params, "bestEffort");
+  const silent = readBooleanParam(params, "silent");
 
   const replyToId = readStringParam(params, "replyTo");
   const threadId = readStringParam(params, "threadId");
@@ -884,6 +885,7 @@ async function handleSendAction(ctx: ResolvedActionContext): Promise<MessageActi
             }
           : undefined,
       abortSignal,
+      silent: silent ?? undefined,
     },
     to,
     message,