feat(web): configurable web search providers with fallback

Add multi-provider web search support: Brave (default), Tavily,
DuckDuckGo, and SearXNG. Falls back to DuckDuckGo when provider
credentials are missing. Providers are dispatched via a map with
register_provider() for plugin extensibility.

- WebSearchConfig with env-var resolution and from_legacy() bridge
- Config migration for legacy flat keys (tavilyApiKey, searxngBaseUrl)
- SearXNG URL validation, explicit error for unknown providers
- ddgs package (replaces deprecated duckduckgo-search)
- 16 tests covering all providers, fallback, env resolution, edge cases
- docs/web-search.md with full config reference

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -150,7 +150,7 @@ nanobot channels login
 
 > [!TIP]
 > Set your API key in `~/.nanobot/config.json`.
-> Get API keys: [OpenRouter](https://openrouter.ai/keys) (Global) · [Brave Search](https://brave.com/search/api/) (optional, for web search)
+> Get API keys: [OpenRouter](https://openrouter.ai/keys) (Global) · [DashScope](https://dashscope.console.aliyun.com) (Qwen) · [Brave Search](https://brave.com/search/api/) or [Tavily](https://tavily.com/) (optional, for web search). SearXNG is supported via a base URL.
 
 **1. Initialize**
 
@@ -185,6 +185,21 @@ Add or merge these **two parts** into your config (other options have defaults).
 }
 ```
 
+**Optional: Web search provider** — set `tools.web.search.provider` to `brave` (default), `duckduckgo`, `tavily`, or `searxng`. See [docs/web-search.md](docs/web-search.md) for full configuration.
+
+```json
+{
+  "tools": {
+    "web": {
+      "search": {
+        "provider": "tavily",
+        "apiKey": "tvly-..."
+      }
+    }
+  }
+}
+```
+
 **3. Chat**
 
 ```bash