Simplify AGENTS.md #5996

Job	Run time
build-and-check	3m 11s
test-integration-packages/internal	1m 8s
test-integration-packages/billing	1m 3s
test-integration-sdk	5m 48s
test-integration-common	54s
test-integration-cli	50s
test-integration-web	44s
test-integration-packages/agent-runtime	46s
test-packages/internal	57s
test-packages/billing	49s
test-sdk	43s
test-packages/agent-runtime	47s
test-cli	52s
test-common	43s
test-web	55s
test-.agents	50s
	21m 0s

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