Support installing lc with Nix and on NixOS

[charnock-fr]

On NixOS, lc does not work after the documented install:

• uv downloads interpreters and wheels built for generic Linux, and NixOS cannot run them without nix-ld.
• If a Nix-provided Python is on PATH, uv picks it instead, and project environments then fail to load manylinux C++ extensions (libstdc++.so.6 missing).
• The sandbox denies every command (Sandbox: every sandboxed command fails on NixOS (/usr/bin/env: Permission denied) #246, fixed by Grant each binary's own ELF loader instead of the FHS ones #247).

Getting a working setup takes NixOS-specific configuration and Nix users on other systems have no Nix package available.

We could ship a Nix flake in this repository:

• a package, installable with
  nix profile install github:LightconeResearch/lightcone-cli;
• a NixOS module, so that programs.lightcone.enable = true; sets up what NixOS needs.

What it would ask of maintainers

• Three new files: flake.nix, flake.lock, and a CI workflow that builds the package.
• After each release, one version string in flake.nix needs bumping, because Nix builds see no git tags. CI fails until it is bumped and prints the value to set.

I'm happy to maintain the Nix side, including flake.lock updates. I have an implementation ready and will open a PR referencing this issue.