Disable motion when reduced-motion preference is enabled

[rezrah]

Summary

Resolves https://github.com/primer/brand/issues/804

Animation is already heavily reduced when this OS setting is enabled, but staggered animation has not been disabled.

This change disables all motion, to ensure a consistent experience for users with this preference enabled.

List of notable changes:

• Turned off transitions entirely in the reset file
• Fixed a reduced-motion related bug in RiverStoryScroll

What should reviewers focus on?

• This decision was made at Site Design Jamz this week. Are there any counter reasons why we would want to keep transitions enabled?

Steps to test:

1. Review the code diff
2. Use this story to test the lack of motion

Contributor checklist:

• [ ] All new and existing CI checks pass
• [ ] Tests prove that the feature works and covers both happy and unhappy paths
• [ ] Any drop in coverage, breaking changes or regressions have been documented above
• [ ] New visual snapshots have been generated / updated for any UI changes
• [ ] All developer debugging and non-functional logging has been removed
• [ ] Related issues have been referenced in the PR description

Reviewer checklist:

• [ ] Check that pull request and proposed changes adhere to our contribution guidelines and code of conduct
• [ ] Check that tests prove the feature works and covers both happy and unhappy paths
• [ ] Check that there aren't other open Pull Requests for the same update/change

Screenshots:

Please try to provide before and after screenshots or videos

Before	After
https://github.com/user-attachments/assets/11b6bfbb-49b0-4fa8-8dc6-87927976912b	https://github.com/user-attachments/assets/efacddfb-9566-4064-87a3-f4cd6c390217

Before	After
	

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: e505b49a849611eb22972692941c216ea38d9904

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

This PR includes changesets to release 6 packages

| Name | Type | | ------------------------ | ----- | | @primer/react-brand | Patch | | @primer/brand-primitives | Patch | | @primer/brand-e2e | Patch | | @primer/brand-fonts | Patch | | @primer/brand-config | Patch | | @primer/brand-storybook | Patch |

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[github-actions[bot]]

🟢 No design token changes found

[github-actions[bot]]

🟢 No visual differences found

Our visual comparison tests did not find any differences in the UI.