refactor(SubNav): Remove sx support from SubNav component

[jonrohan]

Changelog

New

Changed

Removed

Remove sx support from SubNav component

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

Testing & Reviewing

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: 5d20042

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/mcp	Patch
@primer/styled-react	Major

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[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	90.25 KB (+0.12% 🔺)
packages/react/dist/browser.umd.js	90.43 KB (+0.1% 🔺)

[primer-integration]

👋 Hi from github/github! Your integration PR is ready: https://github.com/github/github/pull/396632

[primer-integration]

🟢 golden-jobs completed with status success.

[Copilot]

Pull Request Overview

This PR removes the sx prop support from the SubNav component, replacing the internal BoxWithFallback implementation with standard HTML elements. This is part of a larger migration away from styled-system props to reduce bundle size and improve performance.

Key changes:

• Removes sx prop support from SubNav, SubNavLinks, and SubNavLink components
• Replaces BoxWithFallback wrapper with native HTML elements (nav, div, a)
• Updates TypeScript interfaces to remove SxProp dependencies

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
packages/react/src/SubNav/SubNav.tsx	Main refactor removing sx support and BoxWithFallback usage
packages/react/src/SubNav/SubNav.dev.stories.tsx	Removes development stories demonstrating sx usage
e2e/components/SubNav.test.ts	Removes visual regression tests for sx-related stories
.changeset/proud-chairs-study.md	Documents breaking change for major version release

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