Support Daybreak selection in `codex exec` (#49856)

## What changed

- Add the `daybreak` configuration key, defaulting to `false`, with per-invocation overrides such as `-c daybreak=true`.
- Resolve access programs from the model catalog, preferring `daybreak_blue` over `daybreak_red` when enabled and selecting advertised `standard` access when disabled for signed-in ChatGPT accounts. Require the OpenAI provider, a signed-in ChatGPT account, and a compatible model to enable Daybreak.
- Save the choice for new persistent threads and inherit it on resume and fork, including ephemeral forks. Explicit CLI overrides apply only to the current invocation.

## Testing

Add protocol tests for advertised-program selection and exec integration tests for account and provider restrictions, saved choices, ephemeral forks, and invocation-only overrides.

GitOrigin-RevId: ac955f08b5a891e720cfc5f67fabb7b0f6d32bb8