fix(openrouter): remove litellm_prefix to prevent double-prefixed model names

With custom_llm_provider kwarg handling routing, the openrouter/ prefix
caused model names like anthropic/claude-sonnet-4-6 to become
openrouter/anthropic/claude-sonnet-4-6, which OpenRouter API rejects.

nanobot/providers/registry.py

@@ -98,7 +98,7 @@ PROVIDERS: tuple[ProviderSpec, ...] = (
         keywords=("openrouter",),
         env_key="OPENROUTER_API_KEY",
         display_name="OpenRouter",
-        litellm_prefix="openrouter",  # claude-3 → openrouter/claude-3
+        litellm_prefix="",  # routing handled by custom_llm_provider kwarg; no prefix needed
         skip_prefixes=(),
         env_extras=(),
         is_gateway=True,