fix(design-tokens): replace deprecated Sass slash division with math.div

[danielfsousa]

Summary

• Replace deprecated / division operator with math.div() in generated SCSS rem() and em() functions, fixing Dart Sass 2.0 deprecation warnings for all downstream consumers
• Remove unused yarn.lock — all CI workflows and scripts use npm
• Update package-lock.json matchbox version

Closes #1126

Test plan

• Run npm run build in matchbox and verify design-tokens SCSS output uses math.div()
• Verify downstream projects importing design-tokens no longer see Sass deprecation warnings

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Note

Low Risk
Template-only change to generated SCSS math; no runtime, auth, or data-path impact beyond Sass compatibility.

Overview
Updates the design-tokens SCSS generator so emitted rem() and em() helpers use math.div() instead of slash division, and adds @use "sass:math" in that output. This removes Dart Sass 2.0 deprecation warnings for consumers that import the generated styles.

Includes a patch changeset for @sparkpost/design-tokens and a matchbox version bump in package-lock.json (7.2.4 → 7.2.6).

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