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/outbound-send-service.ts

@@ -34,6 +34,7 @@ export type OutboundSendContext = {
     mediaUrls?: string[];
   };
   abortSignal?: AbortSignal;
+  silent?: boolean;
 };
 
 function extractToolPayload(result: AgentToolResult<unknown>): unknown {
@@ -128,6 +129,7 @@ export async function executeSendAction(params: {
     gateway: params.ctx.gateway,
     mirror: params.ctx.mirror,
     abortSignal: params.ctx.abortSignal,
+    silent: params.ctx.silent,
   });
 
   return {