cleanup: remove legacy backend and npm-app #5033

Job	Run time
build-and-check	2m 4s
test-.agents	47s
test-cli	55s
test-npm-app	38s
test-common	40s
test-packages/billing	37s
test-web	40s
test-integration-.agents	1m 13s
test-packages/internal	43s
test-sdk	42s
test-backend	47s
test-integration-npm-app	34s
test-integration-backend	42s
test-packages/agent-runtime	42s
test-integration-sdk	55s
test-integration-common	39s
test-integration-packages/billing	52s
test-integration-packages/agent-runtime	41s
test-integration-packages/internal	46s
test-integration-web	46s
test-integration-cli	39s
	17m 2s

Provide feedback