cleanup: remove legacy backend and npm-app #5064

Job	Run time
build-and-check	2m 1s
test-integration-backend	34s
test-integration-packages/billing	47s
test-integration-cli	39s
test-integration-sdk	56s
test-integration-common	38s
test-backend	40s
test-integration-.agents	1m 4s
test-integration-packages/agent-runtime	41s
test-integration-packages/internal	46s
test-.agents	37s
test-cli	1m 8s
test-web	44s
test-integration-npm-app	39s
test-sdk	53s
test-packages/internal	42s
test-packages/agent-runtime	50s
test-npm-app	39s
test-common	41s
test-integration-web	39s
test-packages/billing	36s
	16m 54s

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