docs: complete UIUX_BRANDING_HANDOFF for designers

[ulises-jeremias]

Summary

• Expand docs/UIUX_BRANDING_HANDOFF.md from a placeholder to a full designer handoff document, mirroring CNA completeness.
• Document CPA-specific CLI UX: Rich stderr output, questionary autocomplete/checkbox flows, _CATEGORY_PALETTE category badges, error tone, version/banner behavior, and interactive vs CI non-interactive defaults.
• Incorporate working brand notes from docs/BRAND.md and outline audit/work-plan sections for a future UI/UX review pass.

Closes#171

Test plan

• Reviewed document structure against CNA docs/UIUX_BRANDING_HANDOFF.md
• Verified CLI behavior references against cli.py and catalog.py
• Confirmed ASCII-only content (no emoji)
• Maintainer review of handoff completeness for designers

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