Add Nix and nix-darwin integration

[Copilot]

This PR introduces Nix and nix-darwin configuration to the dotfiles repository, allowing users to leverage Nix's reproducible package management alongside the existing Homebrew setup.

Added

• Nix Configuration:

  • Basic flake.nix as the main entry point
  • nix/darwin-configuration.nix for system-wide settings
  • nix/home.nix for user-level configuration via home-manager
  • nix/example-host.nix template for host-specific configurations

• Documentation:

  • Installation instructions for Nix and nix-darwin in README.md
  • Usage instructions and customization guidance
  • Explanation of benefits of using Nix

• Shell Integration:

  • Updated Fish shell configuration to work with Nix

Benefits

• Reproducible environments: Same packages across all machines
• Declarative configuration: System defined as code
• Atomic upgrades and rollbacks: Easy recovery if something breaks
• Coexistence with Homebrew: Users can use both package managers

Usage

After installing Nix:

# Apply the configuration
darwin-rebuild switch --flake .#

Customize by editing the Nix files and updating your hostname in flake.nix.

Fixes #1.

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