docs(ui): document the shared Button API and accessibility contract

[7shep]

Problem

packages/ui contains a shared React Button and tests but no package-level guide. Defaults, variants, exports, and accessibility expectations are not documented near the component.

Suggested level: Beginner
Primary area: packages/ui

Acceptance criteria

• Add packages/ui/README.md with imports and examples for every supported Button prop or variant.
• Document the type="button" default and why it prevents accidental form submission.
• Explain accessible-name, disabled-state, keyboard, and focus expectations.
• Document package test, lint, typecheck, and build commands.
• Explain when a component belongs in packages/ui rather than one application.
• Do not add Storybook, a new component, or a new styling system.
• Formatting and existing UI tests pass.