feat: add Loki log query support to lab-monitoring

Add 3 opt-in Loki tools (query_logs, list_labels, list_label_values)
that are registered when LOKI_URL is configured. Includes Loki HTTP
client, CLI commands (logs, labels), NixOS module option, formatting,
and tests.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

nix/lab-monitoring-module.nix

@@ -33,6 +33,12 @@ in
       description = "Alertmanager base URL.";
     };
 
+    lokiUrl = lib.mkOption {
+      type = lib.types.nullOr lib.types.str;
+      default = null;
+      description = "Loki base URL. When set, enables log query tools (query_logs, list_labels, list_label_values).";
+    };
+
     enableSilences = lib.mkOption {
       type = lib.types.bool;
       default = false;
@@ -104,6 +110,8 @@ in
       environment = {
         PROMETHEUS_URL = cfg.prometheusUrl;
         ALERTMANAGER_URL = cfg.alertmanagerUrl;
+      } // lib.optionalAttrs (cfg.lokiUrl != null) {
+        LOKI_URL = cfg.lokiUrl;
       };
 
       script = let