refactor(flake): add user registry and multi-user host support

Separate user identity (data) from the reusable modules, and let a host
declare any number of users instead of exactly one.

- users/registry.nix: per-user identity (name, email, groups, authorized
  and signing keys) as the single source of identity; no user data is
  hardcoded in the modules.
- mkHost takes a `users` set keyed by username; per-user identity is
  injected into each home config via the `identity` module arg
  (extraSpecialArgs is per-host, so it cannot carry per-user data).
- modules/users.nix builds accounts from the registry; modules/ssh.nix no
  longer defines authorized keys (the registry owns them); home/git.nix
  and home/desktop.nix read `identity`; users/emmathorpe/work.nix drops
  its now-redundant git identity override.
- Restructure the tree: users/, home/, modules/, hosts/, lib/ replace the
  former lyrathorpe/ and system/ layout.
- Add standalone homeConfigurations (the portable subset: shell, git,
  editor, claude) and an exported homeModules output for use on machines
  not managed by this flake, or as an input to other flakes.

Behaviour-preserving for existing hosts: lyrathorpe-mbp and
emmathorpe-edaas evaluate to identical derivations; lyrathorpe-t400,
lyrathorpe-macpro31 and lyrathorpe-rpi5 differ only by de-duplicating a
repeated authorized_keys entry. Fixes the SSH authorized-key leak (one
user's key was applied to every account), the hardcoded default git
identity, and the hardcoded EDaaS linger setting.

modules/common-nixos.nix

@@ -0,0 +1,62 @@
+# Options shared by every NixOS host (laptops and the WSL box). Imported via
+# baseModules in flake.nix. Host- and platform-specific settings stay in the
+# per-machine configs; laptop-only settings live in ./laptop.nix.
+{ pkgs, ... }:
+{
+  time.timeZone = "Europe/London";
+  i18n.defaultLocale = "en_GB.UTF-8";
+
+  # Store hygiene. auto-optimise-store hard-links identical files in the store
+  # after each build (cheap dedupe; NOT a garbage collector -- there is
+  # deliberately no automatic GC timer). The larger download buffer avoids
+  # "buffer full" stalls when fetching big NARs over a fast link.
+  nix.settings.auto-optimise-store = true;
+  nix.settings.download-buffer-size = 134217728; # 128 MiB
+
+  # Extra binary cache for the nix-community toolchain (home-manager, nixvim,
+  # treefmt, ...). Merges with any host-specific caches (e.g. the Asahi cache on
+  # the MBP) rather than replacing them.
+  nix.settings.substituters = [ "https://nix-community.cachix.org" ];
+  nix.settings.trusted-public-keys = [
+    "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs="
+  ];
+
+  # Run dynamically-linked foreign binaries (VS Code remote server, prebuilt
+  # toolchains, language-server downloads) on every NixOS host, not just WSL.
+  programs.nix-ld.enable = true;
+
+  # Minimal system-level CLI available before the home-manager profile loads
+  # (e.g. early boot / rescue). User-level tooling lives in home-manager.
+  environment.systemPackages = with pkgs; [
+    git
+    fastfetch
+  ];
+
+  # Fonts on every host. The Nerd Font carries the powerline/Nerd glyphs the
+  # tmux statusline uses (foot names it explicitly in home/sway.nix); Noto sans +
+  # colour emoji prevent tofu in terminals/TUIs/Firefox -- important on the WSL
+  # box, which does not pull the graphical hosts' default Noto stack. The Mac
+  # installs the Nerd Font via the Darwin config.
+  fonts.packages = with pkgs; [
+    nerd-fonts.jetbrains-mono
+    noto-fonts
+    noto-fonts-color-emoji
+  ];
+  # Map the generic fontconfig families so anything asking for "monospace" gets
+  # the Nerd Font (with emoji fallback), not DejaVu.
+  fonts.fontconfig.defaultFonts = {
+    monospace = [
+      "JetBrainsMono Nerd Font"
+      "Noto Color Emoji"
+    ];
+    sansSerif = [
+      "Noto Sans"
+      "Noto Color Emoji"
+    ];
+    serif = [
+      "Noto Serif"
+      "Noto Color Emoji"
+    ];
+    emoji = [ "Noto Color Emoji" ];
+  };
+}