Add setting to disable startup model announcement banners

Feature Request

When a new model launches (e.g., Fable 5), Claude Code displays a startup banner like:

▎ Meet Fable 5, our newest model for complex, long-running work. Switch anytime with /model.

This banner appears on every new Claude Code instance, which is disruptive and annoying — especially for users who open multiple sessions throughout the day.

There is currently no way to suppress this via settings.json. Please add a setting (e.g., startupAnnouncementsEnabled: false) to disable these banners for users who prefer a clean startup experience.

Expected behavior

A settings.json field (similar to spinnerTipsEnabled) that allows users to opt out of model launch announcements at startup. Ideally, the banner should also only show once (not on every new instance) even without an explicit opt-out.