Enable OpenCode Zen for minimax + kimi; add buffbench agents (#624) #607

Job	Run time
build-freebuff	1m 23s
build-and-smoke-freebuff-windows	2m 14s
e2e-version	1m 5s
e2e-startup	1m 7s
e2e-ads-behavior	1m 14s
e2e-terminal-command	1m 2s
e2e-help-command	1m 6s
e2e-slash-commands	51s
e2e-code-edit	51s
e2e-agent-startup	2m 15s
	13m 8s

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