fix(install): error on mutually exclusive --sdk and --no-sdk flags

Previously passing both silently had --no-sdk win. Exit non-zero with
a clear error to match how other exclusive flag pairs (--global/--local,
--config-dir/--local) are handled.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

bin/install.js

@@ -80,6 +80,11 @@ const hasPortableHooks = args.includes('--portable-hooks') || process.env.GSD_PO
 const hasSdk = args.includes('--sdk');
 const hasNoSdk = args.includes('--no-sdk');
 
+if (hasSdk && hasNoSdk) {
+  console.error(`  ${yellow}Cannot specify both --sdk and --no-sdk${reset}`);
+  process.exit(1);
+}
+
 // Runtime selection - can be set by flags or interactive prompt
 let selectedRuntimes = [];
 if (hasAll) {