fix(nix): remove unmaintained snowfall-lib dep and improve packaging

[PlasmaPower]

Description

I removed the unmaintained snowfall-lib from the flake.nix as its last release was in 2024 and it's now producing warnings abut using the deprecated system config attribute when built with new nixpkgs versions.

While I was at it, I also simplified the nix setup to just put the package in flake.nix, and I replaced the custom scripts/update-nix-version.sh with the standard nix-update tool.

cc @p0ryae since I saw you recently worked on the nix setup in #454. I saw on that PR a comment recommending the prompts cp be replaced with install, but I don't think that works as install doesn't support recursive copying.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Testing

Automated Tests

There's no relevant new automated tests here. I've made sure to preserve the update-nix workflow checking that the nix build works by passing --build to nix-update.

• New features include passing tests in .spec.ts/tsx files
• All existing tests pass (pnpm test:all completes successfully)
• Tests cover both success and error scenarios

Manual Testing

I tested that basic functionality works with nix run ., and nix flake check passes. I wasn't able to test the GitHub Actions flow locally, but the changes there seem straightforward and I tested the nix run nixpkgs#nix-update ... command myself.

• Tested with Ollama
• Tested with OpenRouter
• Tested with OpenAI-compatible API
• Tested MCP integration (if applicable)

Checklist

• Code follows project style guidelines
• Self-review completed
• Documentation updated (if needed) - N/A
• No breaking changes (or clearly documented) - N/A
• Appropriate logging added using structured logging (see CONTRIBUTING.md) - N/A