feat: per-package build + prepublishOnly hook (closes #80)

[mellonis]

Summary

Mirrors turing-machine-js#154 shape so post-machine-js releases stop risking stale-`dist/` publishes, and aligns the release workflow uniformly across both repos.

Discovered just before v6.4.0 publish: `packages/machine/package.json` had no `scripts` block. Manual `npm run build` from root before `npm publish` was the only thing keeping dist current. After a session of edits, `dist/` was 105 minutes older than source — would have shipped stale code without the manual rebuild.

What changes

• `scripts/build-node-entries.mjs` — paths resolve via `REPO_ROOT` from `import.meta.url`, so the script behaves the same from root or from a sub-package. Optional `--package=` filter for per-package builds (only one package in this repo, but matches engine's shape).
• `packages/machine/package.json` — new `scripts` block:
  ```json
  "scripts": {
  "build": "tsc --build --verbose tsconfig.build.json && node ../../scripts/build-node-entries.mjs --package=@post-machine-js/machine",
  "prepublishOnly": "npm run build"
  }
  ```
  `npm publish` (or `lerna publish from-package`) from `packages/machine/` now auto-builds first.
• `lerna.json` — version synced `6.0.0` → `6.4.0` (was stale; hand-edited bumps don't touch `lerna.json`, so it had drifted across all post-machine-js releases since v6.0.0).

After this lands — uniform release workflow across both repos

Step	Command
Bump	`npx lerna version <X.Y.Z> --no-git-tag-version --no-push --yes`
Publish	`npx lerna publish from-package --yes` (from repo root)
GH release	`gh release create vX.Y.Z --target master --title vX.Y.Z --notes-file `

Same commands for both repos; per-repo branch-name conventions remain the only difference (`chore/release-X.Y.Z` for turing-machine-js, `vX-Y-Z` for post-machine-js).

Umbrella docs update

The umbrella `mellonis/machines/CLAUDE.md`'s "Release / delivery process" section will be updated to reflect the uniform flow. Direct push to umbrella master (per the established pattern — umbrella allows direct).

Compatibility

• No source / API changes. Infrastructure only.
• No version bump. Next release picks up the prepublishOnly hook automatically.

Test plan

• `cd packages/machine && rm -rf dist && npm publish --dry-run` triggers prepublishOnly, rebuilds dist, prepares 50.9 kB tarball (15 files, version 6.4.0).
• `npx lerna list --json` from root reports `@post-machine-js/machine` at 6.4.0 — `lerna publish from-package` will publish it.
• `npm test` — 267/267
• `npm run typecheck` / `npm run lint` / `npm run build` from root — all clean