Streamline release tooling: changesets + pkg.pr.new

[JReinhold]

Summary

• migrate the repo from Yarn to pnpm, including lockfile and workflow updates
• replace auto release automation with Changesets + pkg-pr-new
• add release-process guidance for agents and a provenance changeset

What changed

• switched packageManager to pnpm, added pnpm-lock.yaml, removed Yarn artifacts, and updated package/workflow commands
• added .changeset/config.json, a patch changeset, changeset/release scripts, and removed auto + @auto-it/*
• replaced the release workflow with changesets/action, added preview publishing, and enabled npm provenance

Validation

• pnpm install --frozen-lockfile
• pnpm exec prettier --check package.json pnpm-lock.yaml CONTRIBUTING.md AGENTS.md CLAUDE.md src/test-storybook.ts .github/workflows/nightly.yml .github/workflows/release.yml .github/workflows/publish-preview.yml .github/workflows/stress-test.yml .github/workflows/tests-extended.yml .github/workflows/tests.yml .github/workflows/typecheck.yml .changeset/config.json .changeset/kind-wasps-switch.md
• pnpm test-storybook -- --help
• Baseline check on a fresh clone showed the existing pnpm build and pnpm test failures were already present before this migration:
  • pnpm build: DTS error in src/jest-playwright-entries/extends.ts
  • pnpm test: existing failure in src/util/getParsedCliOptions.test.ts