Blankslate: shift size scale and use default button size

[Copilot]

Rescales Blankslate sizes so the compact styling is the default. The old small styles become the default medium, the old medium styles become large, and the old large styles are removed. The primary action button now uses the default (medium) size at every Blankslate size.

Related PR

Companion change: primer/view_components#4110 — Blankslate: make compact styling the default.

Both PRs make the compact Blankslate presentation the default so the component stays visually consistent across our React and Rails/ViewComponents implementations. view_components has no size prop, BlankslateContext, or small/medium/large scale, so the portable part of this change there is simply promoting the compact styling to the base rules and dropping the now-redundant @container responsive block. They should land together to avoid the two implementations drifting apart.

Changelog

New

Changed

• size="medium" (default) now renders the former small styling; size="large" renders the former medium styling.
• Blankslate.PrimaryAction always renders a default-size button instead of a small button on the compact size.
• Updated stories, docs metadata, Figma connect, and padding test to match the new scale.

Removed

• size="small" value and the former large styles.
• Redundant @container responsive block (the compact presentation it produced is now the default).
• Now-unused Blankslate size context (BlankslateContext), which only existed to drive the conditional button size.

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; if selected, include a brief description as to why

Migration: size no longer accepts "small". Consumers using size="small" should drop the prop (default medium now matches the old small visuals). Consumers relying on the old medium/large visuals should bump up one step (medium → large); the old large has no replacement.

Testing & Reviewing

• Verify the Default, SizeMedium, and SizeLarge stories render the expected padding/typography.
• Confirm primary action buttons render at the default size for both medium and large.

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge
• (GitHub staff only) Integration tests pass at github/github-ui (Learn more about how to run integration tests)

[changeset-bot]

🦋 Changeset detected

Latest commit: 6919745

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@primer/react	Major
@primer/mcp	Patch

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[dylanatsmith]

I’d argue this could be a minor version but it is sort of a breaking change so 🤷‍♂️

[github-actions]

⚠️ Action required

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