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/api/handlers/v0/auth/github_oidc.go

@@ -252,8 +252,10 @@ func RegisterGitHubOIDCEndpoint(api huma.API, pathPrefix string, cfg *config.Con
 
 // ExchangeToken exchanges a GitHub OIDC token for a Registry JWT token
 func (h *GitHubOIDCHandler) ExchangeToken(ctx context.Context, oidcToken string) (*auth.TokenResponse, error) {
-	// Validate OIDC token with audience "mcp-registry"
-	claims, err := h.validator.ValidateToken(ctx, oidcToken, "mcp-registry")
+	if h.config.GitHubOIDCAudience == "" {
+		return nil, fmt.Errorf("GitHub OIDC exchange disabled: this deployment has no audience configured")
+	}
+	claims, err := h.validator.ValidateToken(ctx, oidcToken, h.config.GitHubOIDCAudience)
 	if err != nil {
 		return nil, fmt.Errorf("failed to validate OIDC token: %w", err)
 	}