chore(flake): treefmt + deadnix/statix + pre-commit; relocate work module

- treefmt-nix drives `nix fmt` and the formatting check (nixfmt/shfmt/
  prettier; generated files and flake.lock excluded). Replaces the
  bespoke find-based check.
- deadnix and statix as flake checks and pre-commit hooks; deadnix
  ignores module-arg patterns, statix.toml disables the two house-style
  lints (repeated_keys, empty_pattern). Fixed the one real deadnix hit
  (unused overlay arg) and statix hit (use inherit for claude-code).
- git-hooks.nix installs the pre-commit gate via the devShell.
- .editorconfig for the base style.
- Move system/modules/work/default.nix -> lyrathorpe/home/work.nix (it is
  a home-manager module). README gains a Development section; docs
  reformatted by the new formatter.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lyrathorpe/home/work.nix

@@ -1,3 +1,5 @@
+# Home-manager module for the work (EDaaS/WSL) profile: corporate git signing,
+# work toolchain packages and tmux tweaks. Imported only by the work host.
 { pkgs, lib, ... }:
 
 {