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/orchestrator.ts

@@ -8,6 +8,7 @@ import type { ToolPolicyContext } from '../../tools/policy.js';
 import type { Attachment } from '../../channels/types.js';
 import { NativeAgent } from './agent.js';
 import type { ToolUseEvent } from './agent.js';
+import type { OutboundAttachmentCollector } from './attachments.js';
 import { shouldCompact } from '../../context/tokens.js';
 import { compactHistory, type CompactionConfig, type CompactionResult, DEFAULT_COMPACTION_CONFIG } from '../../context/compaction.js';
 import { estimateCost } from '../../models/costs.js';
@@ -91,6 +92,8 @@ export interface OrchestratorConfig {
   memoryStore?: MemoryStore;
   /** Policy context for tool filtering (agent tier, provider). */
   toolPolicyContext?: ToolPolicyContext;
+  /** Collector for outbound attachments queued by tools (e.g. media.send). */
+  attachmentCollector?: OutboundAttachmentCollector;
 }
 
 // ── AgentOrchestrator ─────────────────────────────────────────────────
@@ -139,6 +142,7 @@ export class AgentOrchestrator {
       maxIterations: config.maxIterations,
       onToolUse: config.onToolUse,
       toolPolicyContext: config.toolPolicyContext,
+      attachmentCollector: config.attachmentCollector,
     });
 
     // Set the primary tier on the agent