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/tasks/tasks/tests/tasks.spec.ts

@@ -1,7 +1,7 @@
 import { describe, expect, expectTypeOf, it, vi } from 'vitest'
 import { Context } from 'cordis'
 import { Session, SessionId } from '@deepseek-ai/dsh-session'
-import AgentRegistry from '@deepseek-ai/dsh-agent'
+import AgentRegistry, { AgentMessageId } from '@deepseek-ai/dsh-agent'
 import type { Agent } from '@deepseek-ai/dsh-agent'
 import TaskService, { TaskId } from '@deepseek-ai/dsh-tasks'
 import type { TaskHooks, TaskKind, TaskOutcome, TaskSnapshot, TaskStart } from '@deepseek-ai/dsh-tasks'
@@ -23,10 +23,10 @@ function stubAgent(ctx: Context, rawId: string): Agent {
     session: new Session(id),
     status: 'idle' as const,
     ctx: scopeFiber.ctx,
-    send() {},
-    followup() {},
-    steer() {},
-    inject() {},
+    send: () => AgentMessageId('stub'),
+    followup: () => AgentMessageId('stub'),
+    steer: () => AgentMessageId('stub'),
+    inject: () => AgentMessageId('stub'),
     cancel() {},
     whenIdle() { return Promise.resolve() },
   }