feat: unify web tool config under WebToolsConfig + add web tool toggle controls

- Rename WebSearchConfig references to the new WebToolsConfig root struct that wraps both search config and global proxy settings
- Add 'enable' flag to WebToolsConfig to allow fully disabling all web-related tools (WebSearch, WebFetch) at runtime
- Update AgentLoop and SubagentManager to receive the full web config object instead of separate web_search_config/web_proxy parameters
- Update CLI command initialization to pass the consolidated web config struct instead of split fields
- Change default web search provider from brave to duckduckgo for better out-of-the-box usability (no API key required)

nanobot/config/schema.py

@@ -107,7 +107,7 @@ class GatewayConfig(Base):
 class WebSearchConfig(Base):
     """Web search tool configuration."""
 
-    provider: str = "brave"  # brave, tavily, duckduckgo, searxng, jina
+    provider: str = "duckduckgo"  # brave, tavily, duckduckgo, searxng, jina
     api_key: str = ""
     base_url: str = ""  # SearXNG base URL
     max_results: int = 5
@@ -116,6 +116,7 @@ class WebSearchConfig(Base):
 class WebToolsConfig(Base):
     """Web tools configuration."""
 
+    enable: bool = True
     proxy: str | None = (
         None  # HTTP/SOCKS5 proxy URL, e.g. "http://127.0.0.1:7890" or "socks5://127.0.0.1:1080"
     )