Add GitHub Actions Publish workflow (PSGallery release)

[HeyItsGilbert]

What

Create .github/workflows/publish.yml that:

• Triggers on push of a v* tag (e.g. v0.4.0)
• Runs CI first (reuse ci.yml or inline)
• Publishes to PSGallery using Publish-Module with PSGALLERY_API_KEY repo secret

Notes

• API key needs to be added to repo secrets by org admin before this workflow is useful
• Should only run after CI passes