Add @workflowbuilder/ui + ui-tokens packages (1/3: packages)

[librowski]

Part 1 of 3. Stacked series for the @workflowbuilder/ui work, split for review:

1. this PR - add the package (no consumers touched yet)
2. Migrate consumers to @workflowbuilder/ui (2/3: consumers) #43 - migrate consumers onto it
3. UI Library docs: page-per-component + source-generated tables (3/3) #42 - UI Library documentation

Summary

Brings the overflow-ui Base UI rewrite into the monorepo as the publishable @workflowbuilder/ui package (+ private @workflowbuilder/ui-tokens). Nothing consumes it yet - sdk / demo / ai-studio still build against the published @synergycodes/overflow-ui@1.0.0-beta.27 on this branch; the switch-over is Part 2. This keeps the (large but mostly vendored) package addition reviewable on its own.

Changes

• packages/ui + packages/tokens imported from overflow-ui@base-ui-migration, preserving the multi-entry build, the CSS @layer contract (ui.base / ui.component), combine-css-bundle, and the Style-Dictionary token pipeline (tokens.json → CSS). Dropped decision-logs, the overflow-ui.js back-compat shim, dead deps, and the preview app.
• Aligned with root tooling (eslint / tsconfig extend root), shared deps onto the catalog:, React 18→19, @base-ui/react added to the catalog (pinned 1.4.1).
• build:ui builds tokens → ui; build:lib chains tokens → ui → sdk. dist builds on pnpm install via prepare.
• knip.config.js + CLAUDE.md updated to register the two new workspaces.

Verification

• pnpm install clean; build:lib, build, build:ai-studio green; pnpm -r typecheck green for all workspaces (the only failures are the pre-existing astro check errors in apps/docs/{head,sidebar}.astro, unrelated).
• Most of the file count is the vendored library (packages/ui/**) - low review effort; the build/tooling wiring is the real surface here.

Notes

• @workflowbuilder/ui is a second publishable package - the release tag scheme (v*) needs migrating to scoped tags before publishing it (flagged in CLAUDE.md / packages/sdk/RELEASE.md; maintainer, out of scope).
• No npm publish.

[librowski]

Superseded by the rebranched series (no librowski/ prefix): #44 (packages), #45 (consumers), #46 (docs). Closed automatically when the head branch was renamed.