Clarify AGENTS.md policy on compat shims for breaking changes #2592

Job	Run time
checks / readme-snippets	16s
checks / pre-commit	37s
checks / test (3.10, lowest-direct, windows-latest)	1m 28s
checks / test (3.11, locked, windows-latest)	1m 25s
checks / test (3.14, locked, ubuntu-latest)	1m 58s
checks / test (3.14, lowest-direct, ubuntu-latest)	55s
checks / test (3.13, locked, ubuntu-latest)	49s
checks / test (3.11, lowest-direct, ubuntu-latest)	59s
checks / test (3.14, locked, windows-latest)	2m 40s
checks / test (3.10, locked, ubuntu-latest)	59s
checks / test (3.13, lowest-direct, ubuntu-latest)	56s
checks / test (3.13, locked, windows-latest)	1m 23s
checks / test (3.10, locked, windows-latest)	1m 19s
checks / test (3.12, locked, ubuntu-latest)	57s
checks / test (3.12, lowest-direct, windows-latest)	1m 27s
checks / test (3.10, lowest-direct, ubuntu-latest)	56s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 0s
checks / test (3.11, lowest-direct, windows-latest)	1m 53s
checks / test (3.13, lowest-direct, windows-latest)	1m 29s
checks / test (3.12, locked, windows-latest)	1m 41s
checks / test (3.11, locked, ubuntu-latest)	46s
checks / test (3.14, lowest-direct, windows-latest)	1m 42s
all-green	4s
	27m 39s

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