auth: bind GitHub OIDC token exchange to a per-deployment audience (#1229)

## What this does

The publisher CLI now derives the OIDC audience to request from the
`--registry` URL instead of using a fixed constant, and the server
validates the token's `aud` claim against a new per-deployment env var
(`MCP_REGISTRY_GITHUB_OIDC_AUDIENCE`).

Production sets this env var to
`https://registry.modelcontextprotocol.io` (and per-environment URL
elsewhere), so the deployment only accepts tokens that were requested
for it.

## Compatibility

This is a breaking change for older `mcp-publisher` binaries that still
request the legacy hardcoded audience. After this rolls to a deployment,
users on old `mcp-publisher` will see a clear `invalid audience:
expected …, got […]` error and need to upgrade. Users following the
documented `releases/latest` install pattern get the new binary
automatically.

## Test plan

- [ ] CI green
- [ ] Run a synthetic publish from a real GitHub Actions workflow
against staging after deploy
- [ ] Confirm an older `mcp-publisher` against the same staging deploy
returns the expected `invalid audience` error

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/config/config.go

@@ -17,6 +17,8 @@ type Config struct {
 	EnableAnonymousAuth      bool   `env:"ENABLE_ANONYMOUS_AUTH" envDefault:"false"`
 	EnableRegistryValidation bool   `env:"ENABLE_REGISTRY_VALIDATION" envDefault:"true"`
 
+	GitHubOIDCAudience string `env:"GITHUB_OIDC_AUDIENCE" envDefault:""`
+
 	// OIDC Configuration
 	OIDCEnabled      bool   `env:"OIDC_ENABLED" envDefault:"false"`
 	OIDCIssuer       string `env:"OIDC_ISSUER" envDefault:""`