docs: propose spec-driven development ADR - #229

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5, astral-sh/setup-uv@38f3f104447c67c051c4a08e39b64a148898af3a. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/