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.ts

@@ -51,6 +51,7 @@ type MessageSendParams = {
     mediaUrls?: string[];
   };
   abortSignal?: AbortSignal;
+  silent?: boolean;
 };
 
 export type MessageSendResult = {
@@ -173,6 +174,7 @@ export async function sendMessage(params: MessageSendParams): Promise<MessageSen
       deps: params.deps,
       bestEffort: params.bestEffort,
       abortSignal: params.abortSignal,
+      silent: params.silent,
       mirror: params.mirror
         ? {
             ...params.mirror,