Rename public CSS variables to the wb namespace

[librowski]

Public component overrides still used the legacy --ax-public-* namespace after the generated design tokens moved to --wb-*. This stacked change makes the public override contract consistent before the explicit-type layer.

Stacked on #93.

What changed

• Renamed 524 public CSS custom properties one-to-one from --ax-public-* to --wb-public-* across UI, SDK, demo code, and runtime string references.
• Added no compatibility aliases; this is an intentional breaking change. The .ax-public-* typography utility classes remain unchanged.
• Updated the UI API generator and theming documentation to distinguish generated design tokens, component-local public variables, shared UI variables, and SDK-owned overrides.
• Updated existing release notes and added a major changeset for @workflowbuilder/ui and @workflowbuilder/sdk.

Verification

• pnpm build:lib
• UI and SDK lint/typecheck
• pnpm lint:styles
• pnpm -r test
• pnpm build:docs (212 pages; generated component-variable tables)
• pnpm build
• Prettier, git diff --check, namespace scans, and Changesets status
• Independent diff review with no unresolved findings