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>

go.sum

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