Add OpenCode runner to BuffBench #6483

Job	Run time
build-and-check	3m 14s
test-integration-cli	44s
test-integration-packages/internal	1m 11s
test-common	39s
test-integration-web	52s
test-integration-packages/agent-runtime	45s
test-integration-packages/billing	1m 19s
test-packages/agent-runtime	54s
test-sdk	42s
test-packages/internal	51s
test-web	1m 19s
test-integration-common	52s
test-cli	58s
test-.agents	40s
test-packages/billing	40s
test-integration-sdk	1m 21s
	17m 1s

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