CI, .env loading, NODE_ENV flags, cleanup

[webbertakken]

What

• CI: add GitHub Actions workflow (typecheck + tests) on PRs and pushes to main, Node 18/20/22 via corepack pnpm with cache.
• --production / --development: set NODE_ENV for the wrapped script. Flag beats ambient; passing both errors. Added a -- separator so script args pass through verbatim; envoke flags are never forwarded.
• .env loading (via dotenv): reads .env* from repo root + cwd (deduped). Precedence: real env > flag NODE_ENV > cwd files > root files; within a dir .env.[mode].local > .env.[mode] > .env.local > .env. Real env is never overwritten; mode files load only when NODE_ENV is known.
• Docs: README documents both features; completed ## Todo removed.
• Cleanup: drop stale yarn.lock + .yarnrc.yml, fix prepublishOnly to pnpm, bump 0.2.0.

Testing

• pnpm typecheck && pnpm build && pnpm test green (exit-codes, flags, dotenv suites).
• Workflow smoke-tested locally with act (job succeeded).

Note

0.2.0 is not published in this PR; tagging/publish happens after merge.