docs(test): clarify .env.test Windows requirement

Add explicit comment explaining that dotenvx on Windows requires
this file to exist, even if empty, to prevent CI failures.

Author: jdalton

.env.test

@@ -1,4 +1,8 @@
 # Test environment variables
 # This file is used by the test runner via dotenvx
+#
+# REQUIRED: dotenvx on Windows requires this file to exist, even if empty
+# The test runner (scripts/test.mjs) loads this file before running tests
+#
 # Add any test-specific environment variables below