refactor(openrouter): move litellm kwargs into registry

nanobot/providers/registry.py

@@ -12,7 +12,7 @@ Every entry writes out all fields so you can copy-paste as a template.
 
 from __future__ import annotations
 
-from dataclasses import dataclass
+from dataclasses import dataclass, field
 from typing import Any
 
 
@@ -47,6 +47,7 @@ class ProviderSpec:
 
     # gateway behavior
     strip_model_prefix: bool = False  # strip "provider/" before re-prefixing
+    litellm_kwargs: dict[str, Any] = field(default_factory=dict)  # extra kwargs passed to LiteLLM
 
     # per-model param overrides, e.g. (("kimi-k2.5", {"temperature": 1.0}),)
     model_overrides: tuple[tuple[str, dict[str, Any]], ...] = ()
@@ -106,6 +107,7 @@ PROVIDERS: tuple[ProviderSpec, ...] = (
         detect_by_base_keyword="openrouter",
         default_api_base="https://openrouter.ai/api/v1",
         strip_model_prefix=False,
+        litellm_kwargs={"custom_llm_provider": "openrouter"},
         model_overrides=(),
         supports_prompt_caching=True,
     ),