[codex] Add npm release workflow

[nahiyankhan]

Summary

Adds Ghost-style release plumbing for Madrigal:

• configures Changesets for versioning and changelog generation
• adds npm publish metadata and release scripts
• adds a GitHub Actions release workflow that creates version PRs and publishes with provenance
• documents the release workflow and seeds a changelog

Validation

• pnpm exec biome check package.json .changeset/config.json README.md
• node -e "const fs=require('fs'); const YAML=require('yaml'); YAML.parse(fs.readFileSync('.github/workflows/release.yml','utf8')); console.log('release workflow yaml ok')"
• pnpm exec changeset --version
• pnpm exec tsc -p tsconfig.build.json --pretty false
• pre-push hook: pnpm biome check ., pnpm tsc --noEmit, pnpm vitest run

Note: I did not run the nested dev UI npm build in the clean worktree because npm ci --prefix src/dev/ui --ignore-scripts failed with npm's Exit handler never called error.