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/RPi5/configuration.nix

@@ -0,0 +1,40 @@
+# Raspberry Pi 5 (aarch64) headless server. Two roles, split into submodules:
+# ./docker.nix (Docker host with a network socket) and ./reverse-proxy.nix
+# (native nginx). The raspberry-pi-5 nixos-hardware profile (kernel, firmware,
+# device tree) and key-only sshd (../../modules/ssh.nix) are layered on in the
+# flake host table. Install notes: see ./README.md.
+{ ... }:
+{
+  imports = [
+    ./hardware-configuration.nix
+    ./docker.nix
+    ./reverse-proxy.nix
+  ];
+
+  # Match the flake's nixosConfigurations attribute name so `nh os switch`
+  # (which selects by the local hostname) resolves without an explicit -H flag.
+  networking.hostName = "lyrathorpe-rpi5";
+
+  # Headless server: the Sway desktop is intentionally not set up. modules/sway.nix is
+  # not imported and features.swayDesktop.enable defaults to false (declared in
+  # system/modules/features.nix), so this host keeps plain TTY/SSH login.
+
+  # Raspberry Pi boots via U-Boot + extlinux, not GRUB/systemd-boot. The
+  # raspberry-pi-5 nixos-hardware profile supplies the kernel, firmware and
+  # device tree.
+  boot.loader.grub.enable = false;
+  boot.loader.generic-extlinux-compatible.enable = true;
+
+  # Remote administration. Key-only policy and the authorized key come from
+  # ../../modules/ssh.nix; here we just enable the daemon and open the port.
+  services.openssh.enable = true;
+
+  # Default-deny inbound. Open only SSH here; the Docker and nginx submodules
+  # open their own ports (Docker via a source-restricted nftables rule, nginx
+  # via 80/443). List-valued, so these merge with the submodule definitions.
+  networking.firewall.enable = true;
+  networking.firewall.allowedTCPPorts = [ 22 ];
+
+  # See `man configuration.nix` / the stateVersion docs before changing.
+  system.stateVersion = "26.05";
+}