Pin nixpkgs by committing root flake.lock

[Copilot]

What Changed

The review comment flagged that flake.nix referenced nixpkgs-unstable without a committed root lockfile, making first-time nix develop non-reproducible. This update addresses only that comment by adding the repository-root flake.lock.

• Lockfile added
  • Added /flake.lock at repo root to pin the nixpkgs input used by /flake.nix.
• Scope kept surgical
  • No edits to flake.nix, build logic, or other repo files.

{
  "nodes": {
    "nixpkgs": {
      "locked": {
        "rev": "044bfe75bfe4c7bbe043dc17b5e42ea823b84a09"
      }
    }
  }
}

Why

Without a committed root flake.lock, each environment resolves whatever nixpkgs-unstable points to at that moment, producing drift and dirty working trees. Committing the lockfile makes dev-shell resolution deterministic for contributors and CI.

UI Changes

No UI changes.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

[dasmatus]

Flake.lock added