fix(#321): replace JSON config default clone with structuredClone (#324)

tests/bug-321-config-defaults-clone-strategy.test.cjs

@@ -0,0 +1,23 @@
+'use strict';
+
+const { test } = require('node:test');
+const assert = require('node:assert/strict');
+const configuration = require('../get-shit-done/bin/lib/configuration.cjs');
+
+test('mergeDefaults clones defaults without JSON serialization fragility (#321)', () => {
+  const sentinelKey = '__bug321_bigint_sentinel__';
+  const sentinelValue = BigInt('9007199254740993001');
+
+  configuration.CONFIG_DEFAULTS[sentinelKey] = sentinelValue;
+  try {
+    const merged = configuration.mergeDefaults({});
+    assert.equal(
+      merged[sentinelKey],
+      sentinelValue,
+      'mergeDefaults must preserve non-JSON scalar defaults when cloning'
+    );
+  }
+  finally {
+    delete configuration.CONFIG_DEFAULTS[sentinelKey];
+  }
+});