Bump omp flake to v15.5.12 and provision Nix for Copilot agent runs

[Copilot]

omp on x86_64 Linux/WSL was resolving correctly from Nix store but executed as Bun, indicating a bad upstream artifact in the pinned release. This change updates the flake to a newer upstream release and adds Copilot cloud-agent setup so Nix-based checks can run in agent sessions.

• Release pin + artifact integrity refresh

  • Updated oh-my-pi from 15.3.2 to 15.5.12.
  • Repointed all platform URLs (linux-x64, linux-arm64, darwin-x64, darwin-arm64) to v15.5.12.
  • Replaced all sha256 values with the new release digests.

• Copilot cloud-agent environment bootstrap

  • Added .github/workflows/copilot-setup-steps.yml with required copilot-setup-steps job.
  • Installs Nix via DeterminateSystems/nix-installer-action@v19 and verifies availability with nix --version.

# flake.nix
version = "15.5.12";
"x86_64-linux" = {
  url = "https://github.com/can1357/oh-my-pi/releases/download/v15.5.12/omp-linux-x64";
  sha256 = "sha256-feKqv68rm7G8TQCngBUYUbqZgkXJnsS2AcRRGsHqsr0=";
};