Usage docs for the DOM and native plugins #199

Priority: P3

Context

packages/dom/README.md:27 and packages/native/README.md:27 still say // TODO: Add usage documentation. Users land on these READMEs from npm, and agents copy examples from them.

Do

• Replace each TODO with a usePlugin() setup and one example per matcher group, following packages/sinon/README.md.
• Use one commit per package scope: docs(dom): add usage examples and docs(native): add usage examples.

Done when

Every public matcher in each package's src/lib/ElementAssertion.ts appears in that package's README.

Source: code audit of stackbuilders/assertive-ts at e81a614 (2026-10-13) plus open Vanta/Dependabot findings.