feat(agent): rename InboxItemInfo to AgentMessage with an id; send returns it

Add a branded AgentMessageId assigned to each accepted send message and
returned from send/followup/steer/inject (was void). Rename the inbox
event payload InboxItemInfo to AgentMessage, carrying that id so a caller
can correlate a queued item with its enqueue/dequeue/discard events.

packages/core/agent-loop/tests/inbox.spec.ts

@@ -1,8 +1,9 @@
 import { describe, expect, it } from 'vitest'
+import { AgentMessageId } from '@deepseek-ai/dsh-agent'
 import { Inbox } from '../src/inbox.ts'
 
 function message(text: string) {
-  return { content: [{ type: 'text' as const, text }], source: { kind: 'user' as const }, contexts: [], wakeup: true }
+  return { id: AgentMessageId(text), content: [{ type: 'text' as const, text }], source: { kind: 'user' as const }, contexts: [], wakeup: true }
 }
 
 function resolverPair() {