[codex] Route opencode chat models through Zen #6543

Job	Run time
build-and-check	3m 9s
test-integration-packages/internal	1m 5s
test-integration-packages/billing	1m 2s
test-integration-cli	45s
test-integration-web	49s
test-integration-common	43s
test-integration-sdk	1m 12s
test-integration-packages/agent-runtime	45s
test-common	45s
test-cli	1m 37s
test-packages/agent-runtime	46s
test-packages/internal	48s
test-web	1m 30s
test-.agents	48s
test-sdk	41s
test-packages/billing	41s
	17m 6s

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