Add OpenCode runner to BuffBench #554

Job	Run time
build-freebuff	1m 25s
build-and-smoke-freebuff-windows	2m 44s
e2e-ads-behavior	1m 2s
e2e-help-command	1m 25s
e2e-agent-startup	2m 38s
e2e-startup	1m 1s
e2e-terminal-command	1m 16s
e2e-version	1m 2s
e2e-code-edit	1m 2s
e2e-slash-commands	1m 9s
	14m 44s

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