feat: add outbound attachment support with media.send tool

Introduces OutboundAttachment type on OutboundMessage, an
OutboundAttachmentCollector (push/drain pattern), and a media.send
tool that queues files for outbound delivery. Each channel adapter
(Telegram, Discord, Slack, WhatsApp) sends attachments after the
text reply. Includes 15 tests for collector and tool.

src/backends/native/index.ts

@@ -1,4 +1,5 @@
 export { NativeAgent, type NativeAgentConfig, type ToolUseEvent } from './agent.js';
+export { OutboundAttachmentCollector } from './attachments.js';
 export {
   AgentOrchestrator,
   type OrchestratorConfig,