feat(opencode): add configurable subagent concurrency limit - #51471

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Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/github-script@f28e40c7f34bde8b3046d885e986cb6290c5673b. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/