feat(cli): add entrypoint with --help/--version and exit codes

src/cli.h

@@ -0,0 +1,36 @@
+/*
+ * cli.h - argument parsing for the stupidtools entrypoint.
+ *
+ * Copyright (c) 2026 huntedbytheirs
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#ifndef STUPIDTOOLS_CLI_H
+#define STUPIDTOOLS_CLI_H
+
+#include <stdio.h>
+
+/* Must match AC_INIT version in configure.ac (todo 1: 1.0.0). */
+#define STUPIDTOOLS_VERSION "1.0.0"
+
+#define CLI_EXIT_OK 0
+#define CLI_EXIT_RUNTIME 1
+#define CLI_EXIT_USAGE 2
+
+/* What the caller should do after parsing argv. */
+enum cli_action {
+	CLI_ACTION_RUN,     /* proceed with the parsed buildfile (todo 16+) */
+	CLI_ACTION_HELP,    /* usage already printed to stdout, exit 0 */
+	CLI_ACTION_VERSION, /* caller prints the version line, exit 0 */
+	CLI_ACTION_ERROR    /* error + usage already printed to stderr, exit 2 */
+};
+
+struct cli_opts {
+	const char *program;   /* argv[0] as given (or a default) */
+	const char *buildfile; /* single positional buildfile, NULL if none */
+};
+
+enum cli_action cli_parse(int argc, char **argv, struct cli_opts *opts);
+void cli_print_usage(FILE *stream, const char *program);
+
+#endif /* STUPIDTOOLS_CLI_H */