feat(ai): configurable AI provider (anthropic, gemini, openai-compatible) - #170

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: dorny/test-reporter@v1. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/