Add Nix flake for reproducible builds

[AnthonyRonning]

This PR adds Nix flake support for reproducible builds of the Goose CLI. The flake provides a consistent build environment across different systems and makes it easy for Nix users to build, develop, and run Goose.

What's included

• flake.nix - Main flake configuration that:
  • Dynamically reads package metadata from Cargo.toml files (no hardcoded versions)
  • Builds the goose-cli package with all required dependencies
  • Provides a development shell with cargo tools
  • Configures tests to run with proper HOME/XDG directories
  • Supports both Linux and macOS platforms
• flake.lock - Pinned dependencies for reproducible builds
• .gitignore - Added result symlink exclusion

Testing

✅ CLI build and execution tested on NixOS (Linux/aarch64)

• nix build - Successfully builds the CLI binary
• nix run - CLI runs and executes commands properly
• Tests pass with proper environment configuration
• Development shell (nix develop) works with cargo commands

⚠️ Note: Only the CLI has been tested as my NixOS environment doesn't have a desktop environment. The desktop app would need additional testing, but the flake focuses on the CLI package for now.

Usage

  # Build the CLI
  nix build

  # Run directly
  nix run . -- --help

  # Development shell
  nix develop
  cargo build -p goose-cli

Future improvements

• Desktop app support could be added once tested
• Additional Nix-specific documentation could be added to the main README

[michaelneale]

I think this is fine - nix gets mentioned from time to time and from what little I know, it seems amazing and love that people start to work with it, and this doesn't change any existing builds so why not. Thank you @AnthonyRonning !