Add AGENTS.md and GitHub Copilot instructions #876

Job	Run time
build_windows (3.14)	10m 41s
build_windows (3.13)	8m 46s
build_windows (3.12)	7m 29s
build_linux (3.12)	3m 39s
build_windows (3.11)	7m 49s
build_linux (3.14)	3m 34s
build_linux (3.10)	3m 38s
build_windows (3.10)	9m 22s
build_linux (3.11)	3m 22s
build_linux (3.13)	3m 33s
test_linux (3.14, false, ubuntu-latest)	1m 53s
test_linux (3.10, false, ubuntu-latest)	1m 46s
test_linux (3.12, false, ubuntu-latest)	1m 45s
test_linux (3.13, false, ubuntu-latest)	1m 47s
test_linux (3.11, false, ubuntu-latest)	1m 51s
test_windows (3.13, false, windows-latest)	3m 29s
test_windows (3.14, false, windows-latest)	3m 8s
test_windows (3.12, false, windows-latest)	3m 50s
test_windows (3.11, false, windows-latest)	3m 36s
test_windows (3.10, false, windows-latest)	3m 35s
	1h 28m 33s

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