ci: cross-platform scaffold smoke (macOS + Windows)

[ulises-jeremias]

Problem

CLI CI is Linux-only for scaffold validation. CNA runs cross-platform scaffold jobs (Linux/macOS/Windows, node/bun). CPA already fixed Windows file:// paths (#185) but has no ongoing matrix to catch regressions.

Acceptance criteria

• GitHub Actions matrix: ubuntu-latest, macos-latest, windows-latest
• Each cell: install via uvx (or uv tool) + scaffold fastapi-starter with --no-interactive --no-install (slug or fixture catalog)
• Optional: one addon smoke on Linux only to keep cost down
• Document in docs/MAINTENANCE_CI / TROUBLESHOOTING if platform quirks appear

References

• CNA: test.yml cross-platform-scaffold jobs
• Related: Support Windows local file URLs for template scaffolding #185, [Epic 6] CI/CD maturity — GitHub Actions parity #7