Add automatic template description

[TakodaS]

Currently you need to manually add a description for templates as per the documentation

templates = {
    my-template.description = "This is my template created with Snowfall Lib!";
};

However, it seems easier and more sane to automatically extract the description from ./templates/my-template/flake.nix's description field. This is what this PR does, meaning that you no longer need to add the above description manually.

[anntnzrb]

Implemented with Improvements

I've adopted this feature with enhanced error handling for better robustness.

Implementation

The template description auto-extraction now:

✅ Automatically extracts description from ./templates/<name>/flake.nix
✅ Gracefully handles missing flake.nix files
✅ Gracefully handles missing description fields
✅ Maintains backward compatibility with manual overrides
✅ Preserves all metadata (path, description, etc.)

Key Differences from Original PR

The implementation includes additional safety checks:

create-template-metadata = template: let
  flake-file = template + "/flake.nix";
  has-flake = builtins.pathExists flake-file;
  flake-attrs = if has-flake then import flake-file else {};
  description = flake-attrs.description or null;
in
  { name = ...; path = template; }
  // (if description != null then {inherit description;} else {});

This ensures templates without flake.nix or without a description field won't break.

Usage

Templates can now simply define their description in their own flake.nix:

# templates/my-template/flake.nix
{
  description = "My awesome template!";
  # ... rest of template flake
}

No manual configuration needed in the parent flake!

Commit: anntnzrb/snowfall-lib@f00967f