feat: add treefmt, replace topiary-nu with nufmt
- Add treefmt.toml matching helix formatter settings
- nix: nixfmt -s -w 120
- nu: nufmt
- json: biome with 2-space indent
- Replace topiary-nu with nufmt in helix and opencode configs
- Remove custom topiary-nu package
- Format existing .nu and .nix files with treefmt