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/users.nix

@@ -0,0 +1,44 @@
+# System-level user accounts, built from the identity registry
+# (users/registry.nix). `hostUsers` is the host's user set, threaded by mkHost
+# from the flake host table; `userRegistry` is the global identity table passed
+# as a specialArg. Every account's identity -- description, groups, authorized
+# keys -- comes from its registry entry, so no user data is hardcoded here and a
+# host may declare any number of users.
+{
+  config,
+  pkgs,
+  lib,
+  hostUsers,
+  userRegistry,
+  ...
+}:
+
+{
+  programs.zsh.enable = true;
+
+  users.users = lib.mapAttrs (
+    name: spec:
+    let
+      id = userRegistry.${name};
+    in
+    {
+      isNormalUser = true;
+      home = "/home/${name}";
+      description = id.fullName;
+      inherit (id) extraGroups;
+      openssh.authorizedKeys.keys = id.sshAuthorizedKeys;
+      shell = pkgs.zsh;
+    }
+    # Keep this user's systemd --user instance running without an open login
+    # session (e.g. for home-manager user timers). Only emitted when the host
+    # table opts in, so hosts that don't set it leave linger entirely unmanaged.
+    // lib.optionalAttrs (spec ? linger) { inherit (spec) linger; }
+  ) hostUsers;
+
+  programs.firefox = lib.mkIf (config.features.swayDesktop.enable == true) {
+    enable = true;
+  };
+  programs.thunderbird = lib.mkIf (config.features.swayDesktop.enable == true) {
+    enable = true;
+  };
+}