[codex] Route opencode chat models through Zen #614

Job	Run time
build-and-smoke-freebuff-windows	2m 3s
build-freebuff	1m 15s
e2e-help-command	46s
e2e-startup	1m 13s
e2e-ads-behavior	1m 12s
e2e-agent-startup	59s
e2e-version	1m 10s
e2e-terminal-command	58s
e2e-slash-commands	51s
e2e-code-edit	1m 9s
	11m 36s

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