auth: Add DNS auth (#279)

Fixes #270
Closes #233

Adds support for DNS-based authentication for proving ownership of a
particular domain. E.g. if you can add a TXT record to `anthropic.com`,
you can publish anything at `com.anthropic/*` or `com.anthropic.*`.

Notes:
- Uses Ed25519 signatures with 15-second timestamp windows to prevent
replay attacks
- Supports multiple public keys in DNS TXT records for key rotation
scenarios
- Publisher CLI supports DNS auth via --dns-domain and --dns-private-key
flags
- Includes key generation commands in publisher README for easy setup

## Motivation and Context

This implements DNS-based public/private key authentication to allow
people to actually publish at their own domains. Currently,
authentication requires GitHub accounts which limits publishing to the
io.github.* namespace.

DNS-based auth enables:
- Publishing under your actual domain (e.g., com.anthropic/* for
anthropic.com)
- Independence from GitHub or other external auth providers
- Organizations managing their own keys and access to publishing

## How Has This Been Tested?

- Unit tests cover signature verification, DNS TXT parsing, timestamp
validation, and error cases
- Tested locally by editing my own DNS records
- All existing auth methods covered by passing tests

## 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

## Additional context

Will be adding HTTP challenge auth in a similar way as a separate PR,
e.g. for verifying you own `github.com` by hosting a
`github.com/.well-known/mcp-server-challenge` or similar. Just
mentioning this as we discussed HTTP challenges as closely related to
DNS challenges elsewhere, and want to reassure reviewer that this hasn't
been forgotten with this approach. :) Update: tracking the HTTP auth in
https://github.com/modelcontextprotocol/registry/issues/280

internal/model/model.go

@@ -8,6 +8,8 @@ const (
 	AuthMethodGitHubAT AuthMethod = "github-at"
 	// GitHub Actions OIDC authentication
 	AuthMethodGitHubOIDC AuthMethod = "github-oidc"
+	// DNS-based public/private key authentication
+	AuthMethodDNS AuthMethod = "dns"
 	// No authentication - should only be used for local development and testing
 	AuthMethodNone AuthMethod = "none"
 )