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.

hosts/T400/configuration.nix

@@ -0,0 +1,57 @@
+# ThinkPad T400 (NixOS). Shared laptop options live in ../../modules/laptop.nix;
+# only host-specific settings are here. Install notes (boot variants, GPU,
+# partitions): see ./README.md.
+{ config, ... }:
+
+{
+  imports = [
+    ./hardware-configuration.nix
+    # Boot: import exactly ONE, matching the firmware currently flashed.
+    # Stock Lenovo BIOS and coreboot+SeaBIOS both use boot-bios.nix.
+    ./boot-bios.nix
+    # ./boot-coreboot-grub.nix # coreboot with the GRUB payload (config-only GRUB)
+    # ./boot-coreboot-uefi.nix # coreboot with the Tianocore/edk2 UEFI payload
+    #                          #   (systemd-boot; carries its own ESP mount)
+  ];
+
+  networking.hostName = "T400-NixOS";
+
+  console.font = "Lat2-Terminus16";
+
+  # Low-RAM host (4 GiB max): a compressed RAM swap reduces disk paging.
+  zramSwap.enable = true;
+
+  # This host accepts SSH, so open 22 (the firewall itself is enabled in
+  # laptop.nix with a default-deny policy).
+  services.openssh.enable = true;
+  networking.firewall.allowedTCPPorts = [ 22 ];
+
+  # Intel Core 2 (Penryn) microcode. Redistributable firmware (enabled in
+  # workstation.nix) supplies the iwlwifi blobs (Intel WiFi Link 5100/5300) and
+  # the radeon firmware needed by the discrete GPU below.
+  hardware.cpu.intel.updateMicrocode = true;
+
+  # Battery longevity: cap charging to 75-80%. tlp itself comes from the
+  # nixos-hardware lenovo-thinkpad profile; tp_smapi supplies the threshold
+  # sysfs on this 2008-era ThinkPad (kernel-native natacpi is too new for it).
+  boot.kernelModules = [ "tp_smapi" ];
+  boot.extraModulePackages = [ config.boot.kernelPackages.tp_smapi ];
+  services.tlp.settings = {
+    START_CHARGE_THRESH_BAT0 = 75;
+    STOP_CHARGE_THRESH_BAT0 = 80;
+  };
+
+  # This T400 has the optional discrete GPU fitted: an ATI Mobility Radeon HD
+  # 3470 (RV620), driven by the open `radeon` KMS driver. Load it in the initrd
+  # for early modesetting (clean Sway/Wayland start); firmware comes from
+  # enableRedistributableFirmware above.
+  #
+  # The T400 has switchable graphics (this discrete GPU + the Intel GMA
+  # 4500MHD). Select "Discrete" in the firmware's graphics setting so only the
+  # ATI is live; if you instead run "Integrated", the Intel i915 driver takes
+  # over with no extra config and `radeon` simply stays idle.
+  boot.initrd.kernelModules = [ "radeon" ];
+
+  # See `man configuration.nix` / the stateVersion docs before changing.
+  system.stateVersion = "26.05";
+}