chore: #5 Reference @singleton-sd/tokens in meta/BRAND

[patoperpetua]

Summary

• Extend meta.json colors with tokenId / cssVar and pin @singleton-sd/tokens@1.5.0
• Update BRAND.md mapping table (notes legacy gray500 → token gray-600)
• Add a unit test for the token mapping contract

Closes #5

Test plan

• pnpm test
• pnpm validate
• Spot-check hex still match SVG masters (#FFB300, #C89200, #7F848A)

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Review configuration:

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

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Run ID: 9fc32297-8e79-454b-bcca-7d47fe16435a

📥 Commits

Reviewing files that changed from the base of the PR and between f45cea9 and ae4c4f7.

📒 Files selected for processing (3)

• BRAND.md
• meta.json
• scripts/validate-assets.test.mjs

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