docs: modernize development guidelines and rename to AGENTS.md #1811

Job	Run time
checks / readme-snippets	15s
checks / pre-commit	34s
checks / test (3.12, locked, ubuntu-latest)	58s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 0s
checks / test (3.14, locked, ubuntu-latest)	2m 29s
checks / test (3.13, locked, windows-latest)	3m 16s
checks / test (3.13, locked, ubuntu-latest)	48s
checks / test (3.10, lowest-direct, windows-latest)	2m 39s
checks / test (3.14, lowest-direct, windows-latest)	2m 44s
checks / test (3.13, lowest-direct, windows-latest)	2m 35s
checks / test (3.11, locked, ubuntu-latest)	52s
checks / test (3.10, locked, ubuntu-latest)	55s
checks / test (3.10, locked, windows-latest)	2m 22s
checks / test (3.11, lowest-direct, windows-latest)	2m 57s
checks / test (3.11, lowest-direct, ubuntu-latest)	51s
checks / test (3.10, lowest-direct, ubuntu-latest)	57s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 22s
checks / test (3.12, locked, windows-latest)	4m 23s
checks / test (3.14, locked, windows-latest)	3m 48s
checks / test (3.12, lowest-direct, windows-latest)	3m 38s
checks / test (3.11, locked, windows-latest)	3m 30s
checks / test (3.13, lowest-direct, ubuntu-latest)	57s
all-green	5s
	43m 55s

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