feat: publish module to the Terraform Registry

[gjed]

Background

The `refactor-terraform-module` change (#22) makes `terraform/` a standalone, reusable module consumed via a git ref:

```hcl
source = "github.com/gjed/github-configs-template//terraform?ref=v1.0.0"
```

A git-ref source works but requires consumers to pin a full GitHub URL. Publishing to the Terraform Registry gives a cleaner source address, version constraints, and discoverability.

What needs to happen

• Ensure the repository is public (required by the Registry).

• Rename the repo to match the Registry naming convention: `terraform--` — e.g. `terraform-github-org` (or keep the current name and use a topic to signal intent).

• Add a `README.md` inside `terraform/` — the Registry renders it as the module docs page.

• Tag a release (`v1.0.0`) after feat(terraform): extract reusable publishable module #22 merges — the Registry picks up semver tags automatically.

• Sign in to registry.terraform.io with the GitHub account that owns the repo and publish the module.

• Update the consumer example and wiki to use the Registry source address:

  ```hcl
  source = "gjed/org/github"
  version = "~> 1.0"
  ```

References

• Terraform Registry: Publishing Modules
• feat(terraform): extract reusable publishable module #22 — initial reusable module refactor
• Using as a Module wiki page