cleanup: remove legacy backend and npm-app #5056

Job	Run time
build-and-check	1m 58s
test-backend	42s
test-cli	1m 11s
test-.agents	42s
test-common	41s
test-packages/agent-runtime	50s
test-packages/billing	44s
test-npm-app	39s
test-packages/internal	42s
test-integration-web	41s
test-web	44s
test-integration-.agents	1m 10s
test-integration-cli	59s
test-integration-packages/billing	45s
test-integration-packages/agent-runtime	40s
test-integration-npm-app	42s
test-sdk	47s
test-integration-sdk	55s
test-integration-packages/internal	40s
test-integration-common	44s
test-integration-backend	41s
	17m 37s

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