cleanup: remove legacy backend and npm-app #5057

Job	Run time
build-and-check	1m 58s
test-cli	1m 7s
test-packages/billing	46s
test-.agents	52s
test-packages/internal	37s
test-packages/agent-runtime	51s
test-backend	48s
test-web	45s
test-integration-backend	46s
test-common	47s
test-sdk	46s
test-integration-.agents	1m 8s
test-integration-packages/agent-runtime	48s
test-integration-cli	49s
test-integration-packages/billing	41s
test-integration-common	39s
test-integration-npm-app	40s
test-npm-app	39s
test-integration-packages/internal	46s
test-integration-web	48s
test-integration-sdk	54s
	17m 55s

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