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.

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: bafa104

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

👋 Hi, this pull request contains changes to the source code that github/github-ui depends on. If you are GitHub staff, test these changes with github/github-ui using the integration workflow. If this doesn't work, you can also use the original workflow here. Check the integration testing docs for step-by-step instructions. Or, apply the integration-tests: skipped manually label to skip these checks.

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