auth: Add GitHub OIDC auth (#278)

Adds support for auth'ing to the registry via [GitHub Actions
OIDC](https://docs.github.com/en/actions/concepts/security/openid-connect).

Fixes #271

## Motivation and Context

People will probably want to publish to the registry from CI
environments. DNS auth can work here fine
(https://github.com/modelcontextprotocol/registry/issues/270). But for
people who haven't set up DNS, the standard GitHub device flow won't
really work here. We could make this nice by supporting GitHub Actions
OIDC.

## How Has This Been Tested?

Tested locally with a real GitHub OIDC token from a personal repo.

## Breaking Changes

None

## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [x] New and existing tests pass locally
- [x] I have added appropriate error handling
- [x] I have added or updated documentation as needed

internal/model/model.go

@@ -4,8 +4,10 @@ package model
 type AuthMethod string
 
 const (
-	// GitHub OAuth authentication
-	AuthMethodGitHub AuthMethod = "github"
+	// GitHub OAuth authentication (access token)
+	AuthMethodGitHubAT AuthMethod = "github-at"
+	// GitHub Actions OIDC authentication
+	AuthMethodGitHubOIDC AuthMethod = "github-oidc"
 	// No authentication - should only be used for local development and testing
 	AuthMethodNone AuthMethod = "none"
 )