fix: follow-up for salvaged PR #8952

- Rename provider_contracts.py -> volcengine_byteplus.py for explicitness
- Consolidate duplicate host-to-provider mappings: provider_for_base_url()
  now uses the canonical _URL_TO_PROVIDER from model_metadata.py instead of
  maintaining a separate 20-entry dict
- Add volcengine/byteplus to runtime_provider.py model-dependent base URL
  resolution (kimi-style special case) so manually-edited configs resolve
  the coding-plan base URL correctly
- Remove volcengine/byteplus from _API_KEY_PROVIDER_AUX_MODELS — the
  main-model-first design in _resolve_auto() handles these providers
  already; entries were dead code in the normal flow
- Add VOLCENGINE_API_KEY and BYTEPLUS_API_KEY to OPTIONAL_ENV_VARS in
  config.py so they appear in hermes setup
- Update docs: environment-variables.md, fallback-providers.md,
  configuration.md

website/docs/reference/environment-variables.md

@@ -44,6 +44,8 @@ All variables go in `~/.hermes/.env`. You can also set them with `hermes config
 | `KILOCODE_BASE_URL` | Override Kilo Code base URL (default: `https://api.kilo.ai/api/gateway`) |
 | `XIAOMI_API_KEY` | Xiaomi MiMo API key ([platform.xiaomimimo.com](https://platform.xiaomimimo.com)) |
 | `XIAOMI_BASE_URL` | Override Xiaomi MiMo base URL (default: `https://api.xiaomimimo.com/v1`) |
+| `VOLCENGINE_API_KEY` | Volcengine API key for Doubao / Seed models ([volcengine.com/product/ark](https://www.volcengine.com/product/ark)) |
+| `BYTEPLUS_API_KEY` | BytePlus API key for Seed / Dola models ([byteplus.com/en/product/modelark](https://www.byteplus.com/en/product/modelark)) |
 | `HF_TOKEN` | Hugging Face token for Inference Providers ([huggingface.co/settings/tokens](https://huggingface.co/settings/tokens)) |
 | `HF_BASE_URL` | Override Hugging Face base URL (default: `https://router.huggingface.co/v1`) |
 | `GOOGLE_API_KEY` | Google AI Studio API key ([aistudio.google.com/app/apikey](https://aistudio.google.com/app/apikey)) |