Add GitHub Actions CI for `nix flake check`

[bBlazewavE]

Summary

Add a CI workflow that validates the flake on every push and PR. This catches broken inputs, syntax errors, and evaluation failures before they reach users.

Proposed workflow

• Trigger: push to main, pull requests
• Steps: install Nix (Determinate Systems action), run nix flake check
• Optional: scheduled weekly run to catch upstream breakage

Example

name: CI
on: [push, pull_request]
jobs:
  check:
    runs-on: macos-latest
    steps:
      - uses: actions/checkout@v4
      - uses: DeterminateSystems/nix-installer-action@main
      - run: nix flake check

[bBlazewavE]

✅ Done! I've added GitHub Actions CI for the shellsmith flake.

What was added:

1. Created .github/workflows/ci.yml with two jobs:
   • flake-check: Runs nix flake check on every push and PR
   • flake-build: Builds the dev shell to catch breakage early
2. Scheduled weekly runs to catch upstream nixpkgs-unstable breakage proactively

Workflow triggers:

• Push to main branch
• Pull requests to main branch
• Weekly schedule (Sundays at midnight UTC)

The workflow uses DeterminateSystems/nix-installer-action for fast, reliable Nix setup on macOS runners.