lazily compute agent runtime implementation #3974

Job	Run time
build-and-check	2m 26s
test-integration-.agents	31s
test-integration-common	35s
test-integration-packages/agent-runtime	29s
test-integration-web	34s
test-integration-cli	34s
test-integration-backend	44s
test-integration-npm-app	32s
test-npm-app	30s
test-cli	28s
test-common	34s
test-.agents	41s
test-web	31s
test-backend	36s
test-packages/agent-runtime	41s
	10m 26s

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