add self-hosted web search via SearXNG

new search_web tool calls SearXNG (cluster-internal, free, no tracking)
instead of Mistral's built-in web_search ($0.03/query + rate limits).

returns structured results from DuckDuckGo, Wikipedia, StackOverflow,
GitHub, arXiv, and Brave. no API keys, no cost, no rate limits.

removed Mistral AgentTool::web_search() from orchestrator — replaced
by the custom tool which goes through Sol's normal tool dispatch.

src/agents/definitions.rs

@@ -69,11 +69,7 @@ pub fn orchestrator_request(
         name: ORCHESTRATOR_NAME.to_string(),
         description: Some(ORCHESTRATOR_DESCRIPTION.to_string()),
         instructions: Some(instructions),
-        tools: {
-            let mut all_tools = tools;
-            all_tools.push(AgentTool::web_search());
-            Some(all_tools)
-        },
+        tools: if tools.is_empty() { None } else { Some(tools) },
         handoffs: None,
         completion_args: Some(CompletionArgs {
             temperature: Some(0.5),