docs: add AGENTS.md with fixture workflow and update generated.json #978

Job	Run time
parser-tests (pgsql-cli)	55s
parser-tests (parser)	56s
parser-tests (plpgsql-deparser)	1m 1s
parser-tests (plpgsql-parser)	1m 5s
parser-tests (utils)	54s
parser-tests (transform)	5m 43s
parser-tests (proto-parser)	1m 1s
parser-tests (deparser)	56s
parser-tests (traverse)	1m 0s
	13m 31s

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