feat: make tools parameter optional via R_USE_TOOLS env var check

Add use_tools() helper that reads R_USE_TOOLS environment variable to
conditionally include tools descriptions in chat JSON output, defaulting
to true when unset or unrecognized.

chat.h

@@ -51,7 +51,9 @@ char *chat_json(const char *role, const char *message) {
 
   if (role != NULL && message != NULL) {
     message_add(role, message);
-    json_object_object_add(root_object, "tools", tools_descriptions());
+    if(use_tools()){
+        json_object_object_add(root_object, "tools", tools_descriptions());
+    }
   }
 
   json_object_object_add(root_object, "messages", message_list());

r.h

@@ -32,6 +32,26 @@ char *_model = NULL;
 #define DB_FILE "~/.r.db"
 #define PROMPT_TEMPERATURE 0.1
 
+
+bool use_tools(){
+    if(getenv("R_USE_TOOLS") != NULL){
+        const char *value = getenv("R_USE_TOOLS");
+        if (!strcmp(value, "true")) {
+            return true;
+        }
+        if (!strcmp(value, "false")) {
+            return false;
+        }
+        if (!strcmp(value, "1")) {
+            return true;
+        }
+        if (!strcmp(value, "0")) {
+            return false;
+        }
+    }
+    return true;
+}
+
 char * get_env_system_message(){
     if (getenv("R_SYSTEM_MESSAGE") != NULL) {
         return strdup(getenv("R_SYSTEM_MESSAGE"));