Add AGENTS.md and GitHub Copilot instructions (#276) #881

Job	Run time
build_windows (3.13)	8m 16s
build_windows (3.14)	8m 58s
build_linux (3.13)	3m 43s
build_linux (3.10)	3m 44s
build_linux (3.11)	3m 45s
build_windows (3.12)	8m 24s
build_windows (3.11)	6m 57s
build_linux (3.14)	3m 16s
build_windows (3.10)	8m 52s
build_linux (3.12)	3m 36s
test_linux (3.10, false, ubuntu-latest)	2m 3s
test_linux (3.12, false, ubuntu-latest)	1m 52s
test_linux (3.11, false, ubuntu-latest)	1m 58s
test_linux (3.13, false, ubuntu-latest)	1m 54s
test_linux (3.14, false, ubuntu-latest)	2m 6s
test_windows (3.11, false, windows-latest)	3m 29s
test_windows (3.10, false, windows-latest)	3m 36s
test_windows (3.12, false, windows-latest)	3m 22s
test_windows (3.14, false, windows-latest)	3m 24s
test_windows (3.13, false, windows-latest)	3m 8s
	1h 26m 23s

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