cleanup: remove legacy backend and npm-app #5027

Job	Run time
build-and-check	1m 58s
test-.agents	46s
test-cli	1m 30s
test-packages/agent-runtime	55s
test-packages/internal	40s
test-integration-backend	41s
test-integration-.agents	1m 9s
test-npm-app	43s
test-backend	37s
test-integration-packages/agent-runtime	45s
test-sdk	59s
test-packages/billing	39s
test-integration-npm-app	1m 8s
test-integration-common	45s
test-integration-packages/internal	44s
test-common	40s
test-integration-cli	44s
test-integration-packages/billing	42s
test-web	49s
test-integration-sdk	52s
test-integration-web	38s
	18m 24s

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