docs: modernize development guidelines and rename to AGENTS.md #1812

Job	Run time
checks / pre-commit	34s
checks / readme-snippets	13s
checks / test (3.11, locked, windows-latest)	2m 44s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 26s
checks / test (3.10, locked, windows-latest)	2m 27s
checks / test (3.10, locked, ubuntu-latest)	55s
checks / test (3.10, lowest-direct, ubuntu-latest)	55s
checks / test (3.10, lowest-direct, windows-latest)	2m 39s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 2s
checks / test (3.11, lowest-direct, ubuntu-latest)	51s
checks / test (3.13, lowest-direct, windows-latest)	2m 43s
checks / test (3.12, locked, ubuntu-latest)	56s
checks / test (3.13, locked, ubuntu-latest)	49s
checks / test (3.11, locked, ubuntu-latest)	57s
checks / test (3.14, locked, windows-latest)	3m 31s
checks / test (3.14, lowest-direct, windows-latest)	2m 24s
checks / test (3.11, lowest-direct, windows-latest)	2m 48s
checks / test (3.12, locked, windows-latest)	3m 47s
checks / test (3.13, locked, windows-latest)	2m 18s
checks / test (3.13, lowest-direct, ubuntu-latest)	1m 1s
checks / test (3.12, lowest-direct, windows-latest)	3m 37s
checks / test (3.14, locked, ubuntu-latest)	2m 27s
all-green	3s
	41m 7s

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