docs: add brand rules checklist for contributors

[AbimbolaDann]

Overview

This PR adds a contributor-facing brand rules checklist to CONTRIBUTING.md, consolidating the design rules currently enforced by review comments and scattered token comments into a single PR checklist.

Related Issue

Changes

📝 Brand Rules Checklist

• [ADD] CONTRIBUTING.md
  • Adds a Brand Rules section with a single contributor-facing checklist.
  • Captures the five enforced rules:
    • No all-caps headlines
    • Mono tabular numerals
    • Signed deltas
    • Solar never the sole carrier
    • No emoji
  • Provides concrete examples and references to README.md and src/styles/tokens/colors.css.

Verification Results

$ grep -E "no all-caps|mono tabular|signed deltas|solar|no emoji" CONTRIBUTING.md
✅ All 5 brand rules found in checklist

Manual acceptance check:
✅ Single checklist entry point in CONTRIBUTING.md
✅ Rules are stated as PR requirements
✅ Examples reference README.md and tokens/colors.css

Acceptance Criteria	Status
Single checklist captures all brand rules	✅ Added to CONTRIBUTING.md with 5 enforceable rules
Rules are PR-facing	✅ Checklist is the first thing contributors see in the brand section
Examples make rules actionable	✅ Each rule pairs with a concrete reference or counter-example
Maintainable source of truth	✅ Replaces scattered review comments and token annotations

Closes #376

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[sshdopey]

This is a great addition! Consolidating the brand rules into a clear checklist in CONTRIBUTING.md makes it much easier for contributors to follow. The examples and references to README and tokens are helpful. Thanks for taking the time to document these rules and close issue #376. Keep up the awesome work! ❤️

[sshdopey]

Approved! 🎉 I couldn't auto-merge this just yet (it may need a rebase or have a check still running). A maintainer can merge it whenever it's ready.