Include a system message that an agent was spawned (since the spawn t… #5109

Job	Run time
build-and-check	2m 33s
test-.agents	42s
test-packages/billing	49s
test-integration-common	42s
test-packages/agent-runtime	2m 15s
test-integration-npm-app	40s
test-cli	1m 17s
test-sdk	49s
test-backend	37s
test-packages/internal	38s
test-common	40s
test-integration-cli	39s
test-integration-backend	41s
test-integration-packages/internal	39s
test-integration-web	42s
test-integration-sdk	50s
test-integration-.agents	1m 15s
test-web	52s
test-integration-packages/agent-runtime	47s
test-integration-packages/billing	48s
test-npm-app	41s
	19m 36s

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