feat: add --auto flag to use the available model of "Auto"

[lanternian]

GitHub introduced a weekly quota. When a model’s quota is exhausted you can’t select models manually — selection must be done via Auto (automatic routing). By providing a special auto credential (the Copilot-Session-Token), we can freely request models that are available to Auto.

add --auto flag tobypass the auto-selection routing and provide an auto-available model when the planned models' quota is exhausted.

available models includes: gpt-5-mini, claude-haiku-4.5, gpt-5.4-mini, gpt-5.3-codex, gpt-4.1, gpt-4o

[betaHi]

Thanks @lanternian for the PR, this looks promising.

Could you help add some tests before we merge? I’d like to cover the auto headers, and also the model-selection behavior when the existing Codex model is not in available_models.

We should make sure --auto actually uses an auto-available model instead of silently keeping or falling back to a non-auto model.