Remove support for sx from BranchName component

[llastflowers]

Closes https://github.com/github/primer/issues/5730

❌ Still need to separately update github-ui components to use @primer/styled-react (sx: 3)

Changelog

Removed

• Remove support for sx from the BranchName component

Rollout strategy

• Major release; if selected, include a written rollout or migration plan

We are using the @primer/styled-react component to mitigate removing sx usage. We will make sure usage is updated upstream before merging.

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 (Learn more about how to run integration tests)

[changeset-bot]

🦋 Changeset detected

Latest commit: 15c149e

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

This PR includes changesets to release 3 packages

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

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[github-actions]

👋 Hi, this pull request contains changes to the source code that github/github depends on. If you are GitHub staff, we recommend testing these changes with github/github using the integration workflow. Thanks!

[github-actions]

size-limit report 📦

Path	Size
packages/react/dist/browser.esm.js	90.31 KB (+0.04% 🔺)
packages/react/dist/browser.umd.js	90.37 KB (-0.09% 🔽)

[primer-integration]

👋 Hi from github/github! Your integration PR is ready: https://github.com/github/github/pull/397512

[Copilot]

Pull Request Overview

This PR removes the deprecated sx prop support from the BranchName component in @primer/react as part of the design system's migration away from styled-components. The component is simplified to use standard CSS modules while maintaining a styled-react wrapper for backward compatibility.

Key changes:

• Remove sx prop support and styled-components dependencies from BranchName
• Simplify component implementation to use direct DOM elements
• Add BranchName to styled-react package exports for migration compatibility

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/react/src/BranchName/BranchName.tsx	Removes sx prop support, BoxWithFallback usage, and simplifies to direct component rendering
packages/react/src/BranchName/BranchName.docs.json	Removes sx prop from component documentation
packages/styled-react/src/index.ts	Adds BranchName export for backward compatibility
packages/styled-react/src/tests/snapshots/exports.test.ts.snap	Updates snapshot to include BranchName export
.changeset/strange-peas-kiss.md	Documents breaking change and minor styled-react update

[primer-integration]

🔴 golden-jobs completed with status failure.