Add `diskonaut` to NixOS/nixpkgs

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## Problem

Currently, `diskonaut` in the official Nix packages repository (`nixpkgs`) may be out of date or not maintained with the latest releases from the upstream repository.

Because `diskonaut` publishes prebuilt binaries to GitHub releases and is available on crates.io, it works fine to build from source or use precompiled binaries manually. However, Nix and NixOS users prefer to manage their tooling declaratively. Currently, keeping `diskonaut` up-to-date in a `flake.nix` dev shell or system configuration requires either:

- Maintaining custom derivations
- Manually pinning versions
- Using workarounds that bypass the reproducible, declarative nature of Nix environments

### Proposed solution

Write or update a Nix derivation (using `rustPlatform.buildRustPackage` since this is a Rust-native project) and submit a Pull Request to [`NixOS/nixpkgs`](https://github.com/NixOS/nixpkgs/tree/nixos-unstable). This will allow Nix users to:

- Install it natively via `nix run nixpkgs#diskonaut`
- Include it declaratively in `flake.nix` environments
- Automatically receive updates when nixpkgs is updated
- Use it as a managed system package on NixOS

### Alternatives considered

- Manually managing custom derivations (fragile, not maintainable long-term)
- Building from source on each system (slow, not reproducible across machines)
- Using pre-compiled npm/crate binaries via other package managers (bypasses Nix's reproducibility guarantees)

The derivation should track the latest stable release and ideally support the platforms already available in upstream releases.

Start by reviewing the nixpkgs package conventions and the upstream crates.io and GitHub release metadata for diskonaut. Use rustPlatform.buildRustPackage as proposed, then verify that the derivation builds and exposes diskonaut through nix run nixpkgs#diskonaut. Done means the latest stable release is packaged reproducibly and the derivation is ready for a nixpkgs pull request.

Written by the indexing model from the issue text.