fix(benchmark): parse positional prompt after flags

bin/gstack-model-benchmark

@@ -40,16 +40,40 @@ const ADAPTER_FACTORIES = {
 
 type OutputFormat = 'table' | 'json' | 'markdown';
 
+const CLI_ARGS = process.argv.slice(2);
+const VALUE_FLAGS = new Set(['--models', '--prompt', '--workdir', '--timeout-ms', '--output']);
+
 function arg(name: string, def?: string): string | undefined {
-  const idx = process.argv.findIndex(a => a === name || a.startsWith(name + '='));
+  const idx = CLI_ARGS.findIndex(a => a === name || a.startsWith(name + '='));
   if (idx < 0) return def;
-  const eqIdx = process.argv[idx].indexOf('=');
-  if (eqIdx >= 0) return process.argv[idx].slice(eqIdx + 1);
-  return process.argv[idx + 1];
+  const eqIdx = CLI_ARGS[idx].indexOf('=');
+  if (eqIdx >= 0) return CLI_ARGS[idx].slice(eqIdx + 1);
+  return CLI_ARGS[idx + 1];
 }
 
 function flag(name: string): boolean {
-  return process.argv.includes(name);
+  return CLI_ARGS.includes(name);
+}
+
+function positionalArgs(args: string[]): string[] {
+  const positional: string[] = [];
+  for (let i = 0; i < args.length; i++) {
+    const current = args[i];
+    if (current === '--') {
+      positional.push(...args.slice(i + 1));
+      break;
+    }
+    if (current.startsWith('--')) {
+      const eqIdx = current.indexOf('=');
+      const flagName = eqIdx >= 0 ? current.slice(0, eqIdx) : current;
+      if (eqIdx < 0 && VALUE_FLAGS.has(flagName) && i + 1 < args.length) {
+        i++;
+      }
+      continue;
+    }
+    positional.push(current);
+  }
+  return positional;
 }
 
 function parseProviders(s: string | undefined): Array<'claude' | 'gpt' | 'gemini'> {
@@ -79,7 +103,7 @@ function resolvePrompt(positional: string | undefined): string {
 }
 
 async function main(): Promise<void> {
-  const positional = process.argv.slice(2).find(a => !a.startsWith('--'));
+  const positional = positionalArgs(CLI_ARGS)[0];
   const prompt = resolvePrompt(positional);
   const providers = parseProviders(arg('--models'));
   const workdir = arg('--workdir', process.cwd())!;