cli : Don't clear system prompt when using '/clear' (#20067)

* Enhance /clear command to include system prompt

Add system prompt to messages when clearing chat history.

* Use lambda

tools/cli/cli.cpp

@@ -382,12 +382,15 @@ int main(int argc, char ** argv) {
         modalities += ", audio";
     }
 
-    if (!params.system_prompt.empty()) {
+    auto add_system_prompt = [&]() {
-        ctx_cli.messages.push_back({
+        if (!params.system_prompt.empty()) {
-            {"role",    "system"},
+            ctx_cli.messages.push_back({
-            {"content", params.system_prompt}
+                {"role",    "system"},
-        });
+                {"content", params.system_prompt}
-    }
+            });
+        }
+    };
+    add_system_prompt();
 
     console::log("\n");
     console::log("%s\n", LLAMA_ASCII_LOGO);
@@ -477,6 +480,8 @@ int main(int argc, char ** argv) {
             }
         } else if (string_starts_with(buffer, "/clear")) {
             ctx_cli.messages.clear();
+            add_system_prompt();
+
             ctx_cli.input_files.clear();
             console::log("Chat history cleared.\n");
             continue;