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/laptop.nix

@@ -0,0 +1,31 @@
+# Portable NixOS hosts (X1, MBP-Asahi). Imported from the host table in
+# flake.nix. Shared graphical-workstation settings live in ./workstation.nix;
+# the only laptop-specific bit is the Wi-Fi backend. Mobile home-manager
+# components (battery block, brightness keys) are gated by the `portable` flag
+# threaded through mkHost -- see home/sway.nix.
+{ ... }:
+{
+  imports = [ ./workstation.nix ];
+
+  # Wi-Fi via iwd with its built-in DHCP/network configuration.
+  networking.wireless.iwd = {
+    enable = true;
+    settings.General.EnableNetworkConfiguration = true;
+  };
+
+  # Lid behaviour: suspend on battery, lock on external power (swayidle's
+  # before-sleep hook locks before the suspend completes either way).
+  services.logind.settings.Login = {
+    HandleLidSwitch = "suspend";
+    HandleLidSwitchExternalPower = "lock";
+  };
+
+  # Bluetooth. The Asahi MBP loads Apple's BT firmware (see its host config) and
+  # the T400 has an optional BT module; enable bluez on both, with blueman as the
+  # GUI/tray manager for the Sway session.
+  hardware.bluetooth = {
+    enable = true;
+    powerOnBoot = true;
+  };
+  services.blueman.enable = true;
+}