fix(opencode): coalesce system messages for OpenAI-compatible providers - #42801

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache/restore@0057852bfaa89a56745cba8c7296529d2fc39830, actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5, actions/setup-node@49933ea5288caeca8642d1e84afbd3f7d6820020. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/