fix: add missing function declarations to cli.h

Co-authored-by: aider (deepseek/deepseek-coder) <aider@aider.chat>

Let's do that.cli.h

@@ -0,0 +1,9 @@
+#ifndef CLI_H
+#define CLI_H
+
+#include "engine.h"
+
+int cli_process_line(Engine *engine, const char *line);
+void cli_run(Engine *engine);
+
+#endif /* CLI_H */