cleanup: remove legacy backend and npm-app #5060

Job	Run time
build-and-check	2m 4s
test-integration-cli	47s
test-integration-packages/agent-runtime	40s
test-integration-packages/internal	38s
test-integration-npm-app	44s
test-integration-.agents	1m 12s
test-integration-web	46s
test-integration-backend	40s
test-integration-common	38s
test-common	42s
test-integration-packages/billing	47s
test-npm-app	47s
test-.agents	45s
test-cli	1m 5s
test-backend	39s
test-packages/billing	38s
test-integration-sdk	49s
test-packages/agent-runtime	58s
test-web	49s
test-packages/internal	38s
test-sdk	53s
	17m 39s

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