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/boot-coreboot-uefi.nix

@@ -0,0 +1,17 @@
+# Boot via coreboot's Tianocore/edk2 (UEFI) payload. This turns the T400 into a
+# real UEFI machine, so use systemd-boot. Unlike Apple's firmware, coreboot's
+# UEFI honours EFI variable writes, so canTouchEfiVariables is on.
+#
+# Requires an EFI System Partition. It is declared here so it travels with this
+# boot mode; the generated hardware-configuration.nix should NOT also define
+# /boot. Label the ESP `ESP` at install, or replace with the generated UUID.
+{ ... }:
+{
+  boot.loader.systemd-boot.enable = true;
+  boot.loader.efi.canTouchEfiVariables = true;
+
+  fileSystems."/boot" = {
+    device = "/dev/disk/by-label/ESP";
+    fsType = "vfat";
+  };
+}