Scope azd deployment job to a GitHub Environment

[Arun Sekhar (achandmsft)]

Binds the azd deployment job in azure-dev.yaml to a GitHub production Environment.

Scoping the deploy job to an Environment lets deployment configuration be managed through environment protection rules (required reviewers, branch/tag restrictions, wait timers) and keeps deployment-time configuration grouped per environment. This aligns with the recommended azd CI/CD pipeline setup.

No workflow logic changes - only adds environment: production to the build job.

Note: a production environment may need to be created in repo settings (Settings -> Environments) for the job to run; reviewers can confirm during review.

[Arun Sekhar (achandmsft)]

Closing in favor of #91, which removes the unused AZURE_CREDENTIALS reference at the root instead of scoping the job to an environment.