Restore authorAgentId in chat post command

The merge accidentally dropped the authorAgentId from postChatMessage,
causing the CLI chat test to fail since it asserts the sender ID appears
in the output.

packages/cli/src/commands/chat/post.ts

@@ -3,6 +3,7 @@ import type { SingleResult } from "../../output/index.js";
 import {
   attachAgentNamesToMessages,
   connectChatClient,
+  resolveChatAuthorAgentId,
   toChatCommandError,
   type ChatCommandOptions,
 } from "./shared.js";
@@ -23,6 +24,7 @@ export async function runPostCommand(
     const payload = await client.postChatMessage({
       room,
       body,
+      authorAgentId: resolveChatAuthorAgentId(),
       replyToMessageId: options.replyTo,
     });
     const [message] = await attachAgentNamesToMessages(client, [