[CLOV-793][Maintenance] Deprecate SCSS @import in Backpack Clean up

[IrinaWei]

In this PR:

• Removes the SCSS upgrade script that was used for the migration process

• Updates package.json to remove node-sass dependency and related configuration

• Updates all documentation and references to use the stable bpk-mixins package instead of unstable__bpk-mixins

Remember to include the following changes:

• Ensure the PR title includes the name of the component you are changing so it's clear in the release notes for consumers of the changes in the version e.g [Clover-123][BpkButton] Updating the colour
• README.md (If you have created a new component)
• Component README.md
• Tests
• Accessibility tests
  • The following checks were performed:
    • Ability to navigate using a keyboard only
    • Zoom functionality (Deque University explanation):
      • The page SHOULD be functional AND readable when only the text is magnified to 200% of its initial size
      • Pages must reflow as zoom increases up to 400% so that content continues to be presented in only one column i.e. Content MUST NOT require scrolling in two directions (both vertically and horizontally)
    • Ability to navigate using a screen reader only
• Storybook examples created/updated
• For breaking changes or deprecating components/properties, migration guides added to the description of the PR. If the guide has large changes, consider creating a new Markdown page inside the component's docs folder and link it here

[Copilot]

Pull Request Overview

This PR completes the migration from legacy Sass API to modern Sass API by deprecating SCSS @import usage and removing the deprecated unstable__bpk-mixins package references throughout the codebase.

• Removes the SCSS upgrade script that was used for the migration process
• Updates package.json to remove node-sass dependency and related configuration
• Updates all documentation and references to use the stable bpk-mixins package instead of unstable__bpk-mixins

Reviewed Changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
scripts/scss/upgrade.sh	Removes the migration script that converted @import to @use syntax
packages/package.json	Removes node-sass peer dependency and its optional configuration
packages/bpk-mixins/README.md	Updates import path from unstable__bpk-mixins to bpk-mixins
decisions/modern-sass-api.md	Updates documentation to reflect completed migration to modern Sass API
CONTRIBUTING.md	Updates development guidelines to use bpk-mixins instead of unstable__bpk-mixins
AGENTS.md	Updates AI agent documentation with new import paths
.stylelintignore	Removes the now-deprecated unstable__bpk-mixins from ignore list

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[Copilot]

Corrected capitalization of 'SASS' to 'Sass' to match the official brand name.

Suggested change

	* If you need to add or modify a mixin, do it in `packages/bpk-mixins`, backpack now has formally deprecated `@import` usage and use Modern SASS API in `packages/bpk-mixins`
	* If you need to add or modify a mixin, do it in `packages/bpk-mixins`, backpack now has formally deprecated `@import` usage and use Modern Sass API in `packages/bpk-mixins`

[skyscanner-backpack-bot]

Visit https://backpack.github.io/storybook-prs/4007 to see this build running in a browser.

[skyscanner-backpack-bot]

Visit https://backpack.github.io/storybook-prs/4007 to see this build running in a browser.

[skyscanner-backpack-bot]

Visit https://backpack.github.io/storybook-prs/4007 to see this build running in a browser.

[skyscanner-backpack-bot]

Visit https://backpack.github.io/storybook-prs/4007 to see this build running in a browser.

[IrinaWei]

I remove this file because this file is only used for migration scss to modern scss and currently there is no scripts using it in the code base. It's added in this pr

[skyscanner-backpack-bot]

Visit https://backpack.github.io/storybook-prs/4007 to see this build running in a browser.

The base branch was changed.

[skyscanner-backpack-bot]

Visit https://backpack.github.io/storybook-prs/4007 to see this build running in a browser.