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

README.md

@@ -842,7 +842,7 @@ Config file: `~/.nanobot/config.json`
 
 | Provider | Purpose | Get API Key |
 |----------|---------|-------------|
-| `custom` | Any OpenAI-compatible endpoint (direct, no LiteLLM) | — |
+| `custom` | Any OpenAI-compatible endpoint | — |
 | `openrouter` | LLM (recommended, access to all models) | [openrouter.ai](https://openrouter.ai) |
 | `volcengine` | LLM (VolcEngine, pay-per-use) | [Coding Plan](https://www.volcengine.com/activity/codingplan?utm_campaign=nanobot&utm_content=nanobot&utm_medium=devrel&utm_source=OWO&utm_term=nanobot) · [volcengine.com](https://www.volcengine.com) |
 | `byteplus` | LLM (VolcEngine international, pay-per-use) | [Coding Plan](https://www.byteplus.com/en/activity/codingplan?utm_campaign=nanobot&utm_content=nanobot&utm_medium=devrel&utm_source=OWO&utm_term=nanobot) · [byteplus.com](https://www.byteplus.com) |
@@ -943,7 +943,7 @@ nanobot agent -c ~/.nanobot-telegram/config.json -w /tmp/nanobot-telegram-test -
 <details>
 <summary><b>Custom Provider (Any OpenAI-compatible API)</b></summary>
 
-Connects directly to any OpenAI-compatible endpoint — LM Studio, llama.cpp, Together AI, Fireworks, Azure OpenAI, or any self-hosted server. Bypasses LiteLLM; model name is passed as-is.
+Connects directly to any OpenAI-compatible endpoint — LM Studio, llama.cpp, Together AI, Fireworks, Azure OpenAI, or any self-hosted server. Model name is passed as-is.
 
 ```json
 {
@@ -1120,10 +1120,9 @@ Adding a new provider only takes **2 steps** — no if-elif chains to touch.
 ProviderSpec(
     name="myprovider",                   # config field name
     keywords=("myprovider", "mymodel"),  # model-name keywords for auto-matching
-    env_key="MYPROVIDER_API_KEY",        # env var for LiteLLM
+    env_key="MYPROVIDER_API_KEY",        # env var name
     display_name="My Provider",          # shown in `nanobot status`
-    litellm_prefix="myprovider",         # auto-prefix: model → myprovider/model
-    skip_prefixes=("myprovider/",),      # don't double-prefix
+    default_api_base="https://api.myprovider.com/v1",  # OpenAI-compatible endpoint
 )
 ```
 
@@ -1135,20 +1134,19 @@ class ProvidersConfig(BaseModel):
     myprovider: ProviderConfig = ProviderConfig()
 ```
 
-That's it! Environment variables, model prefixing, config matching, and `nanobot status` display will all work automatically.
+That's it! Environment variables, model routing, config matching, and `nanobot status` display will all work automatically.
 
 **Common `ProviderSpec` options:**
 
 | Field | Description | Example |
 |-------|-------------|---------|
-| `litellm_prefix` | Auto-prefix model names for LiteLLM | `"dashscope"` → `dashscope/qwen-max` |
-| `skip_prefixes` | Don't prefix if model already starts with these | `("dashscope/", "openrouter/")` |
+| `default_api_base` | OpenAI-compatible base URL | `"https://api.deepseek.com"` |
 | `env_extras` | Additional env vars to set | `(("ZHIPUAI_API_KEY", "{api_key}"),)` |
 | `model_overrides` | Per-model parameter overrides | `(("kimi-k2.5", {"temperature": 1.0}),)` |
 | `is_gateway` | Can route any model (like OpenRouter) | `True` |
 | `detect_by_key_prefix` | Detect gateway by API key prefix | `"sk-or-"` |
 | `detect_by_base_keyword` | Detect gateway by API base URL | `"openrouter"` |
-| `strip_model_prefix` | Strip existing prefix before re-prefixing | `True` (for AiHubMix) |
+| `strip_model_prefix` | Strip provider prefix before sending to gateway | `True` (for AiHubMix) |
 
 </details>