feat: integrate Cerebras AI adapter

[lucianchen]

Summary

• replace the Cerebras AI stub with an OpenAI-compatible chat completions request
• add the requested Cerebras model list
• support dry-run short-circuiting, usage token mapping, and status/body excerpts on provider errors

Verification

• pnpm exec vitest run packages/ai/cerebras/src/index.test.ts
• pnpm --filter @profullstack/sh1pt-ai-cerebras typecheck
• git diff --check

Part of #63.