[codex] add private model credential gateway

[wolfiesch]

What changed

• add an optional provider-neutral t4-model-gateway workload and OCI image
• keep the reusable provider credential in that gateway Pod only; session, controller, and cluster-server Pods receive no provider Secret reference
• pin one HTTPS provider origin plus an exact inference-path allowlist, strip caller authority and proxy headers, reject redirects, and bound request size and duration
• reread the projected credential per request so Kubernetes Secret rotation does not put the value in environment variables or require a gateway restart
• wire a ClusterIP Service and fail-closed NetworkPolicies for session-to-gateway and gateway-to-provider traffic
• remove the obsolete session credential-mode fields from Helm, controller configuration, and runtime preflight
• publish, scan, attest, and promote the fourth cluster image through the existing Woodpecker evidence pipeline

Why

Cluster sessions expose arbitrary-code tools, so placing reusable provider credentials in the session workload would make them available to user code. The separate gateway preserves model access while reducing the credential-bearing trust boundary to one hardened workload. Exact path allowlisting also prevents the credential from being used against provider account, file, fine-tuning, billing, or other administrative APIs.

Validation

• pnpm check
• pnpm test:cluster:ci
• pnpm --filter @t4-code/model-gateway test
• pnpm --filter @t4-code/model-gateway typecheck
• go test ./... in packages/cluster-operator with Helm 3.19.0 available
• helm lint deploy/charts/t4-cluster
• broad pnpm test runs: all affected suites passed; two unrelated 5-second integration tests intermittently exceeded their timeout under full concurrent load, then passed in isolation (start-electron.test.ts in 4.6 s for all five cases; runtime-workspace-server.test.ts in 5.0 s for all three cases with its established 30-second fixture timeout)

No cluster deployment, provider Secret, or production configuration is included in this pull request.

[chatgpt-codex-connector]

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