[kumo] remove inherited ghost button background

[ftzi]

Ghost buttons are documented as having no background, but bg-inherit paints the parent background inside the button. On translucent surfaces, this composites the alpha color twice and creates a darker local fill.

This removes the resting background utility while retaining hover:bg-kumo-tint. It also adds a focused regression test for the variant class contract.

Shadcn use this same no-bg-classname pattern for its ghost variant.

Verification:

• pnpm --filter @cloudflare/kumo exec vp test run --project=unit src/components/button/button.test.tsx
• pnpm --filter @cloudflare/kumo build
• pnpm run test
• pnpm run lint
• pnpm run typecheck
• pnpm run format:check

AI disclosure: This change, test, changeset, and pull request description were prepared by an AI coding agent under contributor direction.

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• Reviews
  • bonk has reviewed the change
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• Tests
  • Tests included/updated
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  • Additional testing not necessary because: