CI + npm publish

[llbbl]

Tasks

• GitHub Actions: build + test on push/PR; pandoc-gated integration tests skip cleanly when pandoc is absent.
• Release workflow: publish to npm (public), version via tag; ESM.
• (optional) Docker image bundling pandoc for zero-setup consumers.

Acceptance: CI is green; a tagged release publishes the package.

Depends on: "Package scaffold (TS library)"