cleanup: remove legacy backend and npm-app #5073

Job	Run time
build-and-check	2m 6s
test-integration-npm-app	42s
test-integration-common	41s
test-common	37s
test-integration-backend	38s
test-integration-packages/agent-runtime	41s
test-integration-.agents	1m 18s
test-integration-cli	42s
test-integration-web	40s
test-integration-packages/billing	38s
test-cli	1m 8s
test-integration-sdk	59s
test-npm-app	42s
test-integration-packages/internal	43s
test-backend	44s
test-.agents	42s
test-sdk	51s
test-packages/agent-runtime	52s
test-packages/internal	39s
test-web	45s
test-packages/billing	40s
	17m 28s

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