test: use filepath.Join for GIT_INDEX_FILE path (Copilot review)

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Soph2mo ago

Co-Authored-By: Claude Fable 5 noreply@anthropic.com Claude-Session: https://claude.ai/code/session_012yi3hHGAGepwfrfjPETjGq Entire-Checkpoint: 4521fd749232

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Real Git-invoked Pre-push Hook Integration Tests

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