CI: build + test on macOS, Windows, and Linux

[codymullins]

Problem

There are no GitHub Actions workflows; tests run manually on macOS only. Cross-platform support (the Windows/Linux verification issues) can't stay green without CI.

Current state

• No .github/workflows/ in the repo.
• tools/cover.sh runs locally and gates 100% line coverage on the three core assemblies (+ the interaction assemblies per the interaction-tier plan).

What's needed

• A CI workflow that builds Skyline.slnx and runs Skyline.Tests + Skyline.Gpu.Tests on macOS, Windows, and Linux.
• Run the windowed harness where a display/GPU is available (or a software adapter); document where it can't.
• Wire the coverage gate (tools/cover.sh) on the macOS job.

Acceptance criteria

• PRs run build + headless tests on all three OSes; the coverage gate runs on at least one job.