Enable OpenCode Zen for minimax + kimi; add buffbench agents #606

Job	Run time
build-freebuff	1m 16s
build-and-smoke-freebuff-windows	2m 5s
e2e-agent-startup	2m 4s
e2e-startup	1m 8s
e2e-help-command	1m 24s
e2e-ads-behavior	1m 21s
e2e-version	1m 2s
e2e-slash-commands	1m 1s
e2e-terminal-command	51s
e2e-code-edit	1m 5s
	13m 17s

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