feat(ai): add Hugging Face provider support

- Add huggingface to KnownProvider type
- Add HF_TOKEN env var mapping
- Process huggingface models from models.dev (14 models)
- Use openai-completions API with compat settings
- Add tests for all provider test suites
- Update documentation

fixes #994

packages/coding-agent/docs/providers.md

@@ -62,6 +62,7 @@ pi
 | Vercel AI Gateway | `AI_GATEWAY_API_KEY` |
 | ZAI | `ZAI_API_KEY` |
 | OpenCode Zen | `OPENCODE_API_KEY` |
+| Hugging Face | `HF_TOKEN` |
 | MiniMax | `MINIMAX_API_KEY` |
 | MiniMax (China) | `MINIMAX_CN_API_KEY` |