* docs(nix): refresh NixOS install guide for channels and CI
Align nixos-install.md with current automation: reusable verify-nix on
next/release, npmDepsHash sync on main, devShell vs shell.nix, nix run,
and pinning by branch or app-v tag. Link maintainers to RELEASE_PROCESS.
* docs(nix): steer production installs to release channel
Add a stability note and clarify pinning: prefer the release branch or
`app-v*` tags for everyday use; reserve main/next for contributors.
Document NixOS/Home Manager installation from the upstream flake,
including Cachix substituter/key configuration, apply commands, and pinning.
Also add a README Linux note linking to the new NixOS guide.
cucadmuh