Document the design and brand rules for contributors

[sshdopey]

Context

The brand rules (no all-caps headlines, mono tabular numerals, signed deltas, solar never the sole carrier, no emoji) are enforced by review and scattered across token comments.

Scope

• Collect the brand rules into one contributor-facing checklist.

Acceptance criteria

• A single checklist captures the brand rules a PR must meet.

Pointers

README.md, src/styles/tokens/colors.css.

[akprinciple]

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I will like to get this issue resolved

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