auth: New auth flow (#273)

Fixes #264. Adds endpoints:

```
/v0/auth/github    # exchange a github token for a registry token that gives you access to namespaces io.github.username and io.github.org for your orgs
/v0/auth/none     # get a registry token for namespace io.modelcontextprotocol.anonymous (only for testing/local dev purposes)
```

Updates `/v0/publish` to accept one of these registry tokens.

## Motivation and Context

See #264

## How Has This Been Tested?

- Running locally and getting a token from `/v0/auth/none` and then
publishing to `/v0/publish`
- Updating the integration tests that get a token and then publish, and
them passing

## Breaking Changes

This is a breaking change to the auth API. This is fine for registry
given it's not deployed yet.

## 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)
- [x] Breaking change (fix or feature that would cause existing
functionality to change)
- [x] 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

## Additional context
<!-- Add any other context, implementation notes, or design decisions
-->

---------

Co-authored-by: claude[bot] <209825114+claude[bot]@users.noreply.github.com>
Co-authored-by: adam jones <domdomegg@users.noreply.github.com>

internal/api/server.go

@@ -8,31 +8,28 @@ import (
 
 	"github.com/danielgtaylor/huma/v2"
 	"github.com/modelcontextprotocol/registry/internal/api/router"
-	"github.com/modelcontextprotocol/registry/internal/auth"
 	"github.com/modelcontextprotocol/registry/internal/config"
 	"github.com/modelcontextprotocol/registry/internal/service"
 )
 
 // Server represents the HTTP server
 type Server struct {
-	config      *config.Config
-	registry    service.RegistryService
-	authService auth.Service
-	humaAPI     huma.API
-	server      *http.Server
+	config   *config.Config
+	registry service.RegistryService
+	humaAPI  huma.API
+	server   *http.Server
 }
 
 // NewServer creates a new HTTP server
-func NewServer(cfg *config.Config, registryService service.RegistryService, authService auth.Service) *Server {
+func NewServer(cfg *config.Config, registryService service.RegistryService) *Server {
 	// Create HTTP mux and Huma API
 	mux := http.NewServeMux()
-	api := router.NewHumaAPI(cfg, registryService, authService, mux)
+	api := router.NewHumaAPI(cfg, registryService, mux)
 
 	server := &Server{
-		config:      cfg,
-		registry:    registryService,
-		authService: authService,
-		humaAPI:     api,
+		config:   cfg,
+		registry: registryService,
+		humaAPI:  api,
 		server: &http.Server{
 			Addr:              cfg.ServerAddress,
 			Handler:           mux,