flake.nix's nixpkgs.url/home-manager.url can't source a version string
from variables.nix -- flake input resolution requires a plain string
literal, confirmed empirically (nix flake metadata errors with
"expected a string or a path but got a thunk" otherwise). This script
is the one-command alternative: bump both release branches in flake.nix
via targeted substitution (never a blind repo-wide replace, so it can't
collide with stateVersion strings elsewhere), with an upstream branch
existence check before writing, plus an optional --tooling flag for
codex-maintenance.sh's separately-pinned nixpkgs-fmt/statix fetch.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01La55Nsss8jZ7ZuzUV9mfot
