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/config.rs

@@ -154,6 +154,8 @@ pub struct ServicesConfig {
     pub gitea: Option<GiteaConfig>,
     #[serde(default)]
     pub kratos: Option<KratosConfig>,
+    #[serde(default)]
+    pub searxng: Option<SearxngConfig>,
 }
 
 #[derive(Debug, Clone, Deserialize)]
@@ -166,6 +168,11 @@ pub struct KratosConfig {
     pub admin_url: String,
 }
 
+#[derive(Debug, Clone, Deserialize)]
+pub struct SearxngConfig {
+    pub url: String,
+}
+
 #[derive(Debug, Clone, Deserialize)]
 pub struct VaultConfig {
     /// OpenBao/Vault URL. Default: http://openbao.data.svc.cluster.local:8200