Add Nix-managed Prek hooks

[afermg]

Summary

• add git-hooks.nix and treefmt-nix with Nix-pinned Ruff linting and formatting
• generate and commit a portable .pre-commit-config.yaml pinned to the Ruff version in flake.lock
• let both Prek and pre-commit users run the same hooks without requiring Nix
• add nix run .#update-pre-commit-config and a flake check that detects stale generated configuration
• install the portable hook automatically in nix develop
• use nixos-unstable and uv sync --frozen --all-extras --dev while preserving the existing Python, CUDA, and library setup

Nix users and CI can continue using treefmt through nix fmt and nix flake check; the portable hook gives non-Nix contributors the equivalent Ruff checks.

Validation

• nix flake check
• nix develop --command prek run --all-files
• nix run .#update-pre-commit-config
• commit-time portable Prek hook

Related issue

Closes #121.