[codex] Route opencode chat models through Zen #6541

Job	Run time
build-and-check	2m 25s
test-integration-packages/billing	1m 2s
test-integration-packages/internal	1m 5s
test-integration-common	39s
test-integration-web	39s
test-integration-packages/agent-runtime	39s
test-integration-sdk	1m 26s
test-integration-cli	44s
test-.agents	43s
test-cli	51s
test-packages/internal	56s
test-packages/billing	44s
test-common	44s
test-packages/agent-runtime	47s
test-web	1m 22s
test-sdk	47s
	15m 33s

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