public npm registry support

[seanseannery]

Problem This Solves

Currently users can only install ops cli via curl install script, Homebrew, or manual download. npm is a very popular package manager used by many developers.

Proposal

Phase 1 (this issue — in progress): Add package.json and a postinstall script so users can install directly from the GitHub repo without needing an npm registry account:
```bash
npm install -g github:seanseannery/opsfile
```

Phase 2 (future): Publish opsfile to the public npm registry so users can install without the github: prefix:
```bash
npm install -g opsfile
```
This requires setting up an npm account/org and a publish workflow in CI. Out of scope for Phase 1.

• Update README.md and any other documentation with the new installation directions