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

Start by tracing how SDK sessions expose runtime events, then compare the referenced runtime pull requests and the NotificationService behavior described here. Check how trusted repository settings and user settings are represented in the SDK. Done means applications can receive company announcements with the specified session.info notification format and replacement behavior.

索引モデルが issue の本文から書いたものです。