cleanup: remove legacy backend and npm-app #5025

Job	Run time
build-and-check	1m 57s
test-backend	40s
test-cli	58s
test-npm-app	40s
test-.agents	38s
test-packages/billing	39s
test-web	40s
test-common	37s
test-packages/agent-runtime	46s
test-sdk	44s
test-integration-.agents	1m 14s
test-integration-common	40s
test-packages/internal	40s
test-integration-backend	40s
test-integration-npm-app	38s
test-integration-web	46s
test-integration-packages/agent-runtime	38s
test-integration-packages/billing	38s
test-integration-cli	38s
test-integration-sdk	53s
test-integration-packages/internal	38s
	16m 22s

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