Add ChatGPT personal access token support

[viyatb-oai]

Why

ChatGPT personal access tokens authenticate against the ChatGPT Codex backend, not the Platform Responses endpoint. Treating an at-... token as openai-api-key sends it to the wrong route and fails authentication.

What changed

• add a distinct codex-access-token input and reject ambiguous credential combinations
• hydrate ChatGPT account metadata through AuthAPI before starting Codex
• route PAT-backed requests through the loopback proxy to the ChatGPT Codex endpoint with the required account and FedRAMP context
• keep the token out of Codex's environment and configuration
• preserve the existing Platform API-key, custom Responses endpoint, and Azure paths
• document the new input and update the security guidance
• regenerate dist/main.js

Testing

• corepack pnpm test (27 tests)
• corepack pnpm run check
• git diff --check
• parsed action.yml as YAML
• completed a live PAT smoke test against the ChatGPT Codex endpoint and verified the token was absent from Codex configuration, subprocess environment, outputs, and temporary artifacts