Test: Add file to test main to preview merge

Author: MikeRayMSFT

test-main-to-preview.md

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+# Test File for Main to Preview Validation
+
+This file tests whether the main branch can merge INTO preview branches.
+
+**Purpose**: Verify that stable content from main can flow into preview branches for development.
+
+**Test Date**: January 20, 2026
+**Test Scenario**: Main branch contributing to preview branch
+
+This validates the workflow's unidirectional protection - allowing stable content IN to preview branches while preventing preview content from going OUT.
+
+This file should be deleted after testing.