feat: add Hugging Face as a first-class inference provider (#3419)

Salvage of PR #1747 (original PR #1171 by @davanstrien) onto current main.

Registers Hugging Face Inference Providers (router.huggingface.co/v1) as a named provider:
- hermes chat --provider huggingface (or --provider hf)
- 18 curated open models via hermes model picker
- HF_TOKEN in ~/.hermes/.env
- OpenAI-compatible endpoint with automatic failover (Groq, Together, SambaNova, etc.)

Files: auth.py, models.py, main.py, setup.py, config.py, model_metadata.py, .env.example, 5 docs pages, 17 new tests.

Co-authored-by: Daniel van Strien <davanstrien@gmail.com>

website/docs/user-guide/features/fallback-providers.md

@@ -44,6 +44,7 @@ Both `provider` and `model` are **required**. If either is missing, the fallback
 | MiniMax | `minimax` | `MINIMAX_API_KEY` |
 | MiniMax (China) | `minimax-cn` | `MINIMAX_CN_API_KEY` |
 | Kilo Code | `kilocode` | `KILOCODE_API_KEY` |
+| Hugging Face | `huggingface` | `HF_TOKEN` |
 | Custom endpoint | `custom` | `base_url` + `api_key_env` (see below) |
 
 ### Custom Endpoint Fallback
@@ -161,7 +162,7 @@ When a task's provider is set to `"auto"` (the default), Hermes tries providers
 
 ```text
 OpenRouter → Nous Portal → Custom endpoint → Codex OAuth →
-API-key providers (z.ai, Kimi, MiniMax, Anthropic) → give up
+API-key providers (z.ai, Kimi, MiniMax, Hugging Face, Anthropic) → give up
 ```
 
 **For vision tasks:**