Support company announcements in SDK sessions

## Summary

Add SDK support for Copilot CLI company announcements so applications embedding the SDK can display centrally configured organization or repository messages.

## Runtime behavior

The agent runtime already supports `companyAnnouncements` (originally `launch_messages`):

- User settings can define an array of announcement strings.
- Repository settings can define the same key in `.github/copilot/settings.json` / `.github/copilot/settings.local.json`
- Repository announcements replace user-level announcements when present.
- In the interactive CLI, startup announcements flow through `NotificationService`; its TUI handler emits an ephemeral `session.info` event with `infoType: "notification"` and a message prefixed with `Company announcement:`.
- Repository announcements are similarly emitted after trusted repository settings are loaded.

Relevant runtime work (private repo):

- https://github.com/github/copilot-agent-runtime/pull/2448
- https://github.com/github/copilot-agent-runtime/pull/4134
- https://github.com/github/copilot-agent-runtime/pull/3696

首先跟踪 SDK 会话如何公开运行时事件，然后比较此处所述的相关运行时 pull request 和 NotificationService 的行为。检查受信任的仓库设置和用户设置在 SDK 中是如何表示的。完成的标准是，应用程序能够以指定的 session.info 通知格式和替换行为接收公司公告。

由索引模型根据 Issue 内容生成。