fix: wire up publish workflow using npm trusted publishers

[sharanmoore]

Summary

• Fixes the publish workflow, which has never succeeded, by switching from a long-lived NPM_TOKEN secret to npm Trusted Publishers (OIDC) — no secrets required.
• Adds a Releasing section to the README documenting the one-time setup and publish process.

Action required before merging

Configure the trusted publisher on npmjs.com:

1. Go to the @gradientlabs/client package → Settings → Trusted Publishers → Add trusted publisher.
2. Select GitHub Actions and fill in:
   • Owner: gradientlabs-ai
   • Repository: gradientlabs-nodejs
   • Workflow: publish.yml

Once that's done, pushing a version tag to main will publish to npm automatically.