Add initial Copilot instructions #4996

Job	Run time
Ruby analysis (macos-latest, default)	1m 25s
Ruby analysis (ubuntu-latest, linked)	51s
Ruby analysis (ubuntu-latest, nightly-latest)	50s
Ruby analysis (ubuntu-latest, default)	47s
Ruby analysis (macos-latest, linked)	49s
Ruby analysis (macos-latest, nightly-latest)	59s
	5m 41s

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