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/tests/agent.spec.ts

@@ -4,6 +4,7 @@ import type { Events } from 'cordis'
 import { Session, SessionId } from '@deepseek-ai/dsh-session'
 import AgentRegistry, {
   Agent,
+  AgentMessageId,
   agentEvents,
   agentInterruptReasonOf,
 } from '@deepseek-ai/dsh-agent'
@@ -20,7 +21,7 @@ function stubAgent(rawId: string, overrides: Partial<Agent> = {}): Agent {
     session: new Session(id),
     status: 'idle',
     ctx: new Context(),
-    send() {},
+    send: () => AgentMessageId('stub'),
     cancel() {},
     whenIdle() { return Promise.resolve() },
     ...overrides,

packages/core/agent/tests/invariant.spec.ts

@@ -1,6 +1,6 @@
 import { describe, expect, it } from 'vitest'
 import { Context } from 'cordis'
-import type { Agent } from '@deepseek-ai/dsh-agent'
+import { AgentMessageId, type Agent } from '@deepseek-ai/dsh-agent'
 import * as AgentInvariant from '@deepseek-ai/dsh-agent/invariant'
 import { scopeTarget } from '@deepseek-ai/dsh-scope'
 import InvariantService from '@deepseek-ai/dsh-invariants'
@@ -58,7 +58,7 @@ describe('agent status invariants', () => {
 })
 
 describe('agent inbox invariants', () => {
-  const info = (steering: boolean) => ({ content: [], source: { kind: 'user' as const }, contexts: [], steering, wakeup: true })
+  const info = (steering: boolean) => ({ id: AgentMessageId('m'), content: [], source: { kind: 'user' as const }, contexts: [], steering, wakeup: true })
 
   it('accepts a dequeue and a discard covered by prior enqueues', async () => {
     const ctx = await setup()