chore: adopt Changesets for release management and changelogs

[PaulGrimshaw]

Context

Currently using lerna publish for releases. @changesets/cli is the community standard for independent-versioning monorepos and gives a much better release UX.

Benefits

• Per-package CHANGELOG.md generated automatically
• PR-based release workflow (Changesets bot opens a 'Release PR' aggregating pending changes)
• Works natively with pnpm workspaces
• Integrates with GitHub Actions for automated npm publish

Changes required

• pnpm add -Dw @changesets/cli
• pnpm changeset init
• Add .changeset/config.json
• Add publish.yml GitHub Action (pairs with #[CI issue])
• Document the release process in CONTRIBUTING.md