Add Cerebras provider preset

[aimanmalib]

We support OpenAI-compatible providers via PROVIDER_PRESETS. Adding a new one is a small, self-contained change — great for a first contribution.

Task: add a preset for Cerebras (OpenAI-compatible, bearer auth, base URL https://api.cerebras.ai/v1).

Steps

1. Add a cerebras entry to PROVIDER_PRESETS (see the existing groq entry as a template): base_url, auth_style bearer, a sensible default model, env_key CEREBRAS_API_KEY, env_base CEREBRAS_BASE_URL.
2. Add a preset-resolution test mirroring the existing groq/deepseek cases.
3. Add a row to the Supported Providers table in the README.

Acceptance: tests pass, and provider=cerebras resolves the right base URL + auth style. No client code changes needed.