Publish as private npm package via GitHub Packages

[tchjhnsn]

To make the digital-brain-backend installable by Polity surfaces (Journey, Polity News, etc.), publish it as a private npm package through GitHub Packages.

1. Configure package.json with "name": "@soulpolity/digital-brain-backend" and "publishConfig": { "registry": "https://npm.pkg.github.com" }.
2. Add GitHub Actions workflow to publish on tag push.
3. Configure each surface's package.json to resolve @soulpolity/* from the GitHub Packages registry.

Benefit: surfaces install the backend via npm install rather than copying code.