New ADR-0003 documenting composite action design rationale and Gitea 1.27 constraints. Updated AGENTS.md, README.md, architecture.md, and ci-cd-workflow.md with composite action documentation. CHANGELOG entry. Generated with [Devin](https://devin.ai) Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>