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.

home/default.nix

@@ -0,0 +1,34 @@
+# Base home-manager profile, shared by every host (graphical or headless).
+# Graphical hosts additionally import ./desktop.nix; the work host imports
+# ./work.nix. See the host table in flake.nix.
+{ ... }:
+{
+  imports = [
+    ./shell.nix
+    ./git.nix
+    ./editor.nix
+    ./claude.nix
+  ];
+
+  # Manage the XDG base-directory layout and ~/.config files. Tools above
+  # (bat themes, gh config, ...) write under xdg.configHome; enabling this
+  # makes the paths explicit and consistent across hosts. No regression: the
+  # defaults match the conventional ~/.config, ~/.cache, ~/.local/share.
+  xdg.enable = true;
+
+  # Editor ($EDITOR and $VISUAL) comes from nixvim's defaultEditor (editor.nix).
+  # Round out the rest of the standard env. desktop.nix adds its own Wayland
+  # session vars; home-manager merges the two attrsets, so these do not clash.
+  home.sessionVariables = {
+    PAGER = "less -FRX"; # -F quit-if-one-screen, -R raw colour, -X no clear
+    # Render man pages through bat (themed): col strips backspace overstrike,
+    # bat -l man -p highlights without its own pager decorations.
+    MANPAGER = "sh -c 'col -bx | bat -l man -p'";
+  };
+
+  # Pinned to the release first installed on these hosts, NOT the current
+  # nixpkgs (26.05). stateVersion freezes stateful defaults (file locations,
+  # service data formats) to that release; bumping it silently migrates that
+  # state and can break it. Leave it -- it is intentional, not stale.
+  home.stateVersion = "25.05";
+}