[codex] Route opencode chat models through Zen (#638) #616

Job	Run time
build-and-smoke-freebuff-windows	2m 7s
build-freebuff	1m 21s
e2e-slash-commands	1m 6s
e2e-ads-behavior	1m 13s
e2e-agent-startup	2m 18s
e2e-version	1m 4s
e2e-code-edit	57s
e2e-terminal-command	59s
e2e-startup	1m 1s
e2e-help-command	1m 24s
	13m 30s

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