[codex] Polish npm package structure

[jrkropp]

Summary

• restructure the repo into two npm packages: @jrkropp/codex-js for core runtime/client/server/testing and @jrkropp/codex-js-react for React UI, shadcn exports, and package CSS
• remove the old core React, shadcn, styles, unstable, and upstream mirror public surfaces
• move React/UI dependencies into the React package and keep the core package dependency-light
• replace copied app/mirror source shape with semantic package source directories and prune unused mirror/stub files from publishable source
• add real generated CSS, semantic ESM output, declaration extension fixing, package-boundary tests, and packed-consumer tests
• update examples, README docs, workspace scripts, changeset, and package release checks for the split packages

Validation

• pnpm check

Notes: pnpm check passes. Vite still emits non-fatal warnings for third-party "use client" directives and large example chunks during example builds.

Scope Notes

• Leaves the existing local edits to packages/codex-js/CODEX_PARITY_LEDGER.json, packages/codex-js/CODEX_PARITY_LEDGER.md, and packages/codex-js/MIRROR_MAP.md out of this PR.