Fix formatting drift after rename, and document git-init requirement

Found via an end-to-end test of nix flake init -> rename -> nix flake
check: renaming can reorder `use` statements alphabetically (e.g.
"acme_widget" now sorts before "criterion"), and treefmt's rustfmt
(nixpkgs) disagreed with cargo fmt's rustfmt (fenix toolchain) on
edition-2024 import grouping, so the formatting check failed right
after a clean rename.

- nix/treefmt.nix: pin programs.rustfmt.edition = "2024" to match
  Cargo.toml, so treefmt and cargo fmt agree.
- scripts/rename-project.sh: auto-format (cargo fmt, falling back to
  nix fmt) after renaming, and note that nix commands need git-tracked
  files.
- README.md/CLAUDE.md: document that `nix flake init -t` doesn't
  git-init for you, and every nix command needs `git add` first.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018ALEXkc7pTro7tF1WcUuKb

nix/treefmt.nix

@@ -4,7 +4,12 @@ _: {
     treefmt = {
       projectRootFile = "flake.nix";
 
+      # Edition must match Cargo.toml's `edition = "2024"` — otherwise
+      # treefmt's rustfmt (from nixpkgs) and `cargo fmt`'s rustfmt (from the
+      # fenix toolchain, which reads the edition from Cargo.toml) disagree
+      # on import ordering/grouping and fight each other.
       programs.rustfmt.enable = true;
+      programs.rustfmt.edition = "2024";
       programs.nixfmt.enable = true;
       programs.deadnix.enable = true;
       programs.statix.enable = true;