Add Copilot setup workflow for Nix-based repository bootstrap

[Copilot]

This adds the Copilot setup workflow configuration to the repository by mirroring the setup used in UnstoppableMango/github. The workflow prepares the Nix development environment early so Copilot-driven runs start from a ready shell.

• Workflow added

  • Introduces .github/workflows/copilot-setup-steps.yml
  • Triggers on workflow dispatch and changes to the workflow file itself on push/pull_request

• Repository bootstrap

  • Checks out the repository
  • Installs Nix
  • Configures Cachix in read-only mode for faster environment materialization
  • Pre-builds the dev shell with nix develop

• Action pinning

  • Pins GitHub Actions to commit SHAs, matching the source configuration

- uses: cachix/cachix-action@5f2d7c5294214f71b873db4b969586b980625e71 # v17
  with:
    name: unstoppablemango
    skipPush: true

- name: Pre-build Nix dev shell
  run: nix develop --command echo "Nix dev shell ready"

[Copilot]

Pull request overview

Adds a dedicated “Copilot Setup Steps” GitHub Actions workflow to bootstrap this Nix-based repository so Copilot-driven runs can start from a materialized dev shell.

Changes:

• Introduces .github/workflows/copilot-setup-steps.yml with workflow_dispatch plus path-scoped push/pull_request triggers.
• Installs Nix, configures Cachix in read-only mode, and pre-builds the dev shell via nix develop.

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