feat: OpenAI agent (#416)

apps/docs/docs/examples/agent.mdx

apps/docs/docs/modules/agent/_category_.yml

apps/docs/docs/modules/agent/index.md

apps/docs/docs/modules/agent/openai.mdx

examples/agent/openai.ts

packages/core/src/OutputParser.ts

packages/core/src/Response.ts

packages/core/src/agent/index.ts

packages/core/src/agent/openai/base.ts

packages/core/src/agent/openai/types/chat.ts

packages/core/src/agent/openai/utils.ts

packages/core/src/agent/openai/worker.ts

packages/core/src/agent/runner/base.ts

packages/core/src/agent/runner/types.ts

packages/core/src/agent/types.ts

packages/core/src/agent/utils.ts

packages/core/src/engines/chat/types.ts

packages/core/src/index.ts

packages/core/src/llm/LLM.ts

packages/core/src/llm/types.ts