Support subscription accounts (e.g. ChatGPT/Codex) with Coder Agents for better pricing

The Problem

Coder Agents does not natively support authentication with subscription accounts which are, for example, $20/mo subscriptions to a specific AI provider. These may have usage caps, but are great predictable ways to use lots of AI. This is because we do not properly support the OAuth sign-in flow to delegate and refresh the short-lived session token.

Because of this, many users find themselves paying much more in API pricing (e.g. $331 in a week vs fixed $20/mo) in tokens when using Coder Agents.

Solution

For AI providers that support external harnesses on subscription (e.g. OpenAI Codex), we should add a "sign in" flow (similar to OpenCode so that a user can use their subscription accounts.

Alternatively, for those not looking to use Coder Agents, see: coder/coder#26864