[FEATURE]: Native Fireworks AI login (browser OAuth) in auth login / /connect

I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Fireworks AI is already in the provider list, but auth login / /connect only offer a manual API-key prompt for it. Other providers (GitHub Copilot, OpenAI) get a native browser sign-in via a built-in auth plugin.

I'd like the same for Fireworks: a "Log in with Fireworks Connect" option that opens the Fireworks SSO in the browser (PKCE + localhost callback), mints an API key for the account, and stores it as a credential automatically. The flow already exists in Fireworks' own open-source CLI (fw-ai/fireconnect, MIT), so the protocol is stable and documented.

Benefit: one-step onboarding for Fireworks users instead of hunting for an API key in the web console.

I have a working implementation as a built-in auth plugin (packages/opencode/src/plugin/fireworks.ts, ~2 files touched) with tests, verified end-to-end against a live Fireworks account. Happy to open a PR if the approach sounds good.