Release and publish both packages independently, with docs #44

Part of the core extraction (#35). Release and publish the two packages with independent versions, and document the core.

Scope

• Release Please: one version and changelog per package; the core starts at 0.1.0, the adapter continues from 0.4.0. A core release also releases the adapter with its dependency range moved.
• A publish script (core before adapter, skips versions npm already has) and release:check.
• A README for each package, docs/ADAPTERS.md, the logos.

Done when

• test:release and release:check pass in CI, and the README's "Releases" section lists the steps to publish the core for the first time.