feat: GitHub Actions CI/CD — build, test, Docker publish, Pi 5 deploy

[Fortinbra]

Spec

This issue tracks the implementation of GitHub Actions workflows for DungeonMaster.

Spec file: \docs/specs/2-github-actions.md\

Mirror the BudgetExperiment CI/CD pattern: CI workflow (build + test + coverage), Docker multi-arch build-and-publish to GHCR (amd64 + arm64 for Pi 5), and release workflow with git-cliff changelog.

Acceptance Gate

Green CI on every PR, Docker images published to GHCR on tag, Pi 5 deployment triggered on successful publish.

[Fortinbra]

✅ Implemented and verified.

Commit: f8f074e (CI/CD workflows + Dockerfiles)

Verification:

• ✓ CI workflow (.github/workflows/ci.yml) exists
• ✓ Docker build-publish workflow exists with multi-arch support (amd64 + arm64)
• ✓ Release workflow with git-cliff exists
• ✓ Dockerfile and Dockerfile.prebuilt use chiseled runtime image
• ✓ docker-compose.pi.yml configured for Pi 5 deployment
• ✓ coverlet.runsettings and cliff.toml in place

Spec status: Done