Add GitHub Actions CI to test builds

[Copilot]

Adds automated CI to validate that the Neovim configuration builds successfully on every push and pull request.

Changes

• .github/workflows/ci.yml: GitHub Actions workflow that:
  • Installs Nix with flakes support
  • Builds the neovim package (nix build .#neovim)
  • Runs flake checks (nix flake check) to validate helper scripts

Triggers on pushes to main and all pull requests.

Original prompt

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This section details on the original issue you should resolve

<issue_title>test build in CI (mandatory)</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes test build in CI (mandatory) #97

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