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/tools/mod.rs

@@ -3,6 +3,7 @@ pub mod devtools;
 pub mod identity;
 pub mod research;
 pub mod room_history;
+pub mod web_search;
 pub mod room_info;
 pub mod script;
 pub mod search;
@@ -197,6 +198,9 @@ impl ToolRegistry {
             tools.extend(identity::tool_definitions());
         }
 
+        // Web search (SearXNG — free, self-hosted)
+        tools.push(web_search::tool_definition());
+
         // Research tool (depth 0 — orchestrator level)
         if let Some(def) = research::tool_definition(4, 0) {
             tools.push(def);
@@ -329,6 +333,13 @@ impl ToolRegistry {
                     anyhow::bail!("Identity (Kratos) integration not configured")
                 }
             }
+            "search_web" => {
+                if let Some(ref searxng) = self.config.services.searxng {
+                    web_search::search(&searxng.url, arguments).await
+                } else {
+                    anyhow::bail!("Web search not configured (missing [services.searxng])")
+                }
+            }
             "research" => {
                 if let (Some(ref mistral), Some(ref store)) = (&self.mistral, &self.store) {
                     anyhow::bail!("research tool requires execute_research() — call with room + event_id context")