feat(nix): saturate builds, add community cache, pin nixpkgs registry

- Drop auto-optimise-store: slows every build for modest disk savings.
  Run 'nix store optimise' manually if disk pressure ever shows up.
- max-jobs=auto, cores=0: defaults are 1/1, which left most of the box
  idle during large closures (LLVM, protobuf, …).
- Add nix-community.cachix.org as an extra substituter with its public
  key. Big hit-rate boost against nixos-unstable, which is what the new
  user registry points 'nixpkgs' at.
- dot_config/nix/registry.json pins 'nixpkgs' indirect ref to
  github:NixOS/nixpkgs/nixos-unstable, so 'nix shell nixpkgs#foo' is
  fast + reproducible. Project flakes are unaffected — they pin their
  own inputs via flake.lock.

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