Add npm publish workflow and prepublish build step

[that-github-user]

Summary

package.json specifies "bin": { "thinktank": "./dist/cli.js" } but:

1. dist/ is gitignored (correct)
2. No prepublishOnly script to build before publishing
3. No GitHub Actions workflow to automate npm releases

Users who npm install -g thinktank will get a broken install because dist/ doesn't exist.

Proposed fix

package.json

"scripts": {
  "prepublishOnly": "npm run build"
}

.github/workflows/release.yml

Trigger on version tags, build, and publish to npm.

Acceptance criteria

• npm pack produces a working package with dist/ included
• GitHub Action publishes to npm on v* tags
• README install instructions work

[that-github-user]

Fixed in #42. Added prepublishOnly script (build + test), files field limiting published package to dist/README/LICENSE, and GitHub Actions release workflow that triggers on v* tags with npm provenance and auto-generated GitHub Release notes.