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/agent.ts

@@ -6,6 +6,7 @@ import type { ToolExecutor } from '../../tools/executor.js';
 import type { ToolResult } from '../../tools/types.js';
 import type { ToolPolicyContext } from '../../tools/policy.js';
 import type { Attachment } from '../../channels/types.js';
+import type { OutboundAttachmentCollector } from './attachments.js';
 import { buildUserMessage, getMessageText } from '../../models/media.js';
 
 export interface ToolUseEvent {
@@ -25,6 +26,8 @@ export interface NativeAgentConfig {
   onToolUse?: (event: ToolUseEvent) => void;
   /** Policy context for tool filtering (agent tier, provider). */
   toolPolicyContext?: ToolPolicyContext;
+  /** Collector for outbound attachments queued by tools (e.g. media.send). */
+  attachmentCollector?: OutboundAttachmentCollector;
 }
 
 // Internal message type for the tool loop — supports both text and structured content blocks.
@@ -47,6 +50,7 @@ export class NativeAgent {
   private _totalUsage: TokenUsage = { inputTokens: 0, outputTokens: 0 };
   private _callCount: number = 0;
   private _toolPolicyContext?: ToolPolicyContext;
+  private _attachmentCollector?: OutboundAttachmentCollector;
 
   constructor(config: NativeAgentConfig) {
     this.modelClient = config.modelClient;
@@ -57,6 +61,7 @@ export class NativeAgent {
     this.maxIterations = config.maxIterations ?? 10;
     this.onToolUse = config.onToolUse;
     this._toolPolicyContext = config.toolPolicyContext;
+    this._attachmentCollector = config.attachmentCollector;
   }
 
   private get history(): Message[] {
@@ -241,4 +246,12 @@ export class NativeAgent {
   getToolPolicyContext(): ToolPolicyContext | undefined {
     return this._toolPolicyContext;
   }
+
+  setAttachmentCollector(collector: OutboundAttachmentCollector | undefined): void {
+    this._attachmentCollector = collector;
+  }
+
+  getAttachmentCollector(): OutboundAttachmentCollector | undefined {
+    return this._attachmentCollector;
+  }
 }