use functions from brand.yml package

[bowerth]

use has_brand and brand_pluck from brand.yml package

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[Copilot]

Pull request overview

Updates the brand.yml Shiny example to rely on the public API from the {brand.yml} package instead of bslib’s internal helpers, improving long-term compatibility as internals change.

Changes:

• Add an explicit {brand.yml} install check and attach the package in the example app.
• Replace bslib::: internal brand_* helper calls with brand.yml::brand_has() / brand.yml::brand_pluck().

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[gadenbuie]

Thanks! I think we can drop the brand.yml:: prefix (it's clear where brand_pluck() comes from). Can you also please add an entry in NEWS.md?

[gadenbuie]

Thank you!