cleanup: remove legacy backend and npm-app #5029

Job	Run time
build-and-check	2m 4s
test-npm-app	39s
test-packages/agent-runtime	51s
test-.agents	41s
test-backend	1m 2s
test-common	39s
test-packages/internal	47s
test-packages/billing	46s
test-cli	57s
test-sdk	48s
test-integration-.agents	1m 9s
test-integration-npm-app	50s
test-integration-packages/agent-runtime	40s
test-web	42s
test-integration-cli	37s
test-integration-sdk	55s
test-integration-common	38s
test-integration-packages/billing	38s
test-integration-web	40s
test-integration-packages/internal	41s
test-integration-backend	38s
	17m 22s

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