Document --agent override behavior #5257

Job	Run time
build-and-check	2m 47s
test-integration-cli	38s
test-integration-packages/billing	45s
test-integration-.agents	38s
test-integration-common	41s
test-integration-packages/agent-runtime	41s
test-cli	1m 10s
test-.agents	40s
test-integration-packages/internal	41s
test-packages/agent-runtime	49s
test-integration-web	38s
test-integration-sdk	40s
test-packages/billing	38s
test-sdk	46s
test-web	43s
test-packages/internal	37s
test-common	40s
	14m 12s

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