cleanup: remove legacy backend and npm-app #5067

Job	Run time
build-and-check	2m 2s
test-backend	46s
test-cli	1m 13s
test-.agents	41s
test-common	41s
test-packages/agent-runtime	48s
test-npm-app	40s
test-packages/internal	45s
test-integration-.agents	1m 13s
test-web	46s
test-sdk	53s
test-packages/billing	47s
test-integration-common	38s
test-integration-cli	46s
test-integration-backend	42s
test-integration-packages/agent-runtime	40s
test-integration-web	42s
test-integration-packages/billing	40s
test-integration-sdk	1m 1s
test-integration-packages/internal	40s
test-integration-npm-app	37s
	17m 41s

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