Add AGENTS.md and GitHub Copilot instructions #852

Job	Run time
build_windows (3.14)	9m 14s
build_linux (3.14)	3m 47s
build_windows (3.10)	8m 46s
build_windows (3.13)	8m 55s
build_windows (3.12)	11m 6s
build_linux (3.12)	3m 27s
build_linux (3.11)	3m 40s
build_windows (3.11)	13m 53s
build_linux (3.13)	3m 37s
build_linux (3.10)	3m 44s
test_linux (3.11, false, ubuntu-latest)	2m 3s
test_linux (3.10, false, ubuntu-latest)	2m 17s
test_linux (3.12, false, ubuntu-latest)	2m 42s
test_linux (3.14, false, ubuntu-latest)	2m 15s
test_linux (3.13, false, ubuntu-latest)	2m 21s
test_windows (3.14, false, windows-latest)	3m 13s
test_windows (3.11, false, windows-latest)	3m 37s
test_windows (3.12, false, windows-latest)	3m 27s
test_windows (3.10, false, windows-latest)	3m 20s
test_windows (3.13, false, windows-latest)	3m 45s
	1h 39m 9s

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