fix: publish Madrigal under Block npm scope

[nahiyankhan]

Summary

• Renames the npm package from the occupied unscoped madrigal name to @block/madrigal.
• Updates README examples, Changesets docs, and changelog heading to use the scoped package name.
• Keeps the CLI binary name as madrigal.

Root Cause

The Release workflow reached npm publish --access public --provenance and failed with npm E403 because the unscoped madrigal package already exists on npm under another maintainer. Publishing under @block/madrigal avoids that name collision and matches the GitHub org/repo ownership.

Verification

• pnpm build
• npm pack --dry-run --ignore-scripts --access public --cache=/private/tmp/madrigal-npm-cache --loglevel=notice
• pnpm check
• pnpm typecheck
• pnpm test
• pre-commit hook: biome check .
• pre-push hook: pnpm vitest run, pnpm biome check ., pnpm tsc --noEmit

Note

The next Release workflow still requires MADRIGAL_NPM_PUBLISH_TOKEN to have publish permission for the @block npm scope.