refactor: replace litellm with native openai + anthropic SDKs

- Remove litellm dependency entirely (supply chain risk mitigation)
- Add AnthropicProvider (native SDK) and OpenAICompatProvider (unified)
- Merge CustomProvider into OpenAICompatProvider, delete custom_provider.py
- Add ProviderSpec.backend field for declarative provider routing
- Remove _resolve_model, find_gateway, find_by_model (dead heuristics)
- Pass resolved spec directly into provider — zero internal lookups
- Stub out litellm-dependent model database (cli/models.py)
- Add anthropic>=0.45.0 to dependencies, remove litellm
- 593 tests passed, net -1034 lines

nanobot/config/schema.py

@@ -249,8 +249,7 @@ class Config(BaseSettings):
         if p and p.api_base:
             return p.api_base
         # Only gateways get a default api_base here. Standard providers
-        # (like Moonshot) set their base URL via env vars in _setup_env
-        # to avoid polluting the global litellm.api_base.
+        # resolve their base URL from the registry in the provider constructor.
         if name:
             spec = find_by_name(name)
             if spec and (spec.is_gateway or spec.is_local) and spec.default_api_base: