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/ssh.nix

@@ -0,0 +1,14 @@
+# Key-only SSH hardening, imported by the hosts that run sshd (T400, Mac Pro,
+# RPi5). The host config still does `services.openssh.enable = true` and opens
+# port 22 next to where it documents the listening service; this module only
+# tightens the policy so a host opting into sshd cannot accidentally ship
+# password/root login. Authorized keys are owned per-user by the identity
+# registry (users/registry.nix, applied via modules/users.nix), not here.
+{ ... }:
+{
+  services.openssh.settings = {
+    PasswordAuthentication = false; # keys only
+    KbdInteractiveAuthentication = false; # no keyboard-interactive fallback
+    PermitRootLogin = "no";
+  };
+}