Add NixOS eval/build GitHub Actions and unignore .github

[moons-14]

Motivation

• Add CI to evaluate and build NixOS configurations from the repository flake to catch breakage early using GitHub Actions.
• Ensure the workflows are tracked by repository by allowing the .github/ directory through .gitignore.

Description

• Add /.github/workflows/nixos-eval.yml which discovers hosts with nix eval and runs a per-host matrix job that evaluates the system derivation (.config.system.build.toplevel.drvPath).
• Add /.github/workflows/nixos-build.yml which discovers hosts with nix eval and runs a per-host matrix job that builds the system with nix build " .#nixosConfigurations.${{ matrix.host }}.config.system.build.toplevel" and prints build logs.
• Both workflows install Nix via cachix/install-nix-action@v31 with experimental-features = nix-command flakes and set accept-flake-config = true, and they run on pull_request to main and workflow_dispatch.
• Update .gitignore to include !.github/ so the new workflow files are committed and not ignored.

Testing

• No automated tests were executed as part of this change; the new workflows will run automatically on future PRs or when triggered manually via workflow_dispatch.

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Codex Task