Add an agent instruction file (AGENTS.md + CLAUDE.md)

Priority: P1. Do this early so later agent tasks can use it.

Context

The audit scored "agent instruction files" 0/4, and agents already contribute to this repo.

Create a root AGENTS.md and a CLAUDE.md that points to it. Run every command before you document it. Cover:

Setup: corepack enable && yarn install (Yarn 4.1.0, Node per .nvmrc).
Verify: yarn compile && yarn lint && yarn test (~30 s). For one package: cd packages/<pkg> && yarn test.
Layout: core expect() dispatch is in packages/core/src/lib/expect.ts. Plugins register via usePlugin() into packages/core/src/lib/config/Config.ts. There's one *Assertion.ts per type.
Matcher pattern to copy: error + invertedError + this.execute({ assertWhen, ... }), JSDoc with @example, and tests that assert both the direct and the .not path with exact messages.
Test rule: every comparison matcher needs an equal-value/boundary case and a negative-number case.
Commit scopes table: all, core, dom, native, sinon.
Dependency-fix recipe: add a resolutions entry, yarn dedupe, then check with yarn why <pkg> and yarn npm audit --all --recursive.
Off-limits: yarn release, npm publish, .github/workflows/release.yml, .yarn/releases/.

Every command in the file runs successfully and its output matches the description.

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