Streamline release tooling: changesets + pkg.pr.new

[JReinhold]

Summary

• migrate the repo from Yarn to pnpm, including lockfile/config/workflow updates
• replace auto with Changesets, add pkg.pr.new preview publishing, and enable npm provenance
• add release-process agent docs plus the required patch changeset

Validation

• pnpm exec prettier . --check
• pnpm lint
• pnpm typecheck
• pnpm exec vitest run --coverage --reporter=dot
• pnpm build
• pnpm install --frozen-lockfile

Org-specific notes

• release workflow now uses the GitHub App token pattern and expects vars.STORYBOOK_BOT_APP_ID plus secrets.STORYBOOK_BOT_APP_PRIVATE_KEY
• release publishing relies on GitHub OIDC (id-token: write) with publishConfig.provenance: true
• no Slack release notification replacement was added