feat(llm): configurable analyzer fan-out concurrency via SKILLSPECTOR_MAX_LLM_CONCURRENCY (part of #303) - #305

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/