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>

deploy/pkg/k8s/registry.go

@@ -185,6 +185,15 @@ func DeployMCPRegistry(ctx *pulumi.Context, cluster *providers.ProviderInfo, env
 									Name:  pulumi.String("MCP_REGISTRY_OIDC_PUBLISH_PERMISSIONS"),
 									Value: pulumi.String("*"),
 								},
+								&corev1.EnvVarArgs{
+									Name: pulumi.String("MCP_REGISTRY_GITHUB_OIDC_AUDIENCE"),
+									Value: pulumi.String(func() string {
+										if environment == "prod" {
+											return "https://registry.modelcontextprotocol.io"
+										}
+										return "https://" + environment + ".registry.modelcontextprotocol.io"
+									}()),
+								},
 							},
 							// StartupProbe protects the DB-retry budget in cmd/registry/main.go:
 							// 30 × 5s = 150s allowed before liveness/readiness take over, which