Add OpenCode runner to BuffBench #6482

Job	Run time
build-and-check	3m 11s
test-integration-packages/billing	1m 5s
test-integration-sdk	1m 23s
test-integration-packages/agent-runtime	40s
test-integration-cli	49s
test-packages/agent-runtime	44s
test-integration-web	50s
test-integration-common	47s
test-packages/billing	45s
test-common	1m 0s
test-integration-packages/internal	1m 14s
test-packages/internal	49s
test-web	1m 17s
test-.agents	58s
test-cli	50s
test-sdk	42s
	17m 4s

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