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.

users/registry.nix

@@ -0,0 +1,40 @@
+# User identity registry -- pure data, no machinery. Keyed by username.
+#
+# Each entry describes WHO a user is (display name, email, authorized/signing
+# keys, supplementary groups). The reusable modules read this indirectly:
+#   - system: modules/users.nix builds users.users.* from `userRegistry`
+#     (specialArg) restricted to the host's `hostUsers` set.
+#   - home:   home/git.nix and home/desktop.nix read the per-user `identity`
+#     module arg, which mkHost derives from the matching registry entry.
+#
+# The login name is injected by mkHost as `identity.username`, so it is not
+# repeated inside each entry here.
+{
+  lyrathorpe = {
+    fullName = "Lyra Thorpe";
+    email = "iam@emmathe.dev";
+    extraGroups = [
+      "wheel"
+      "docker"
+    ];
+    sshAuthorizedKeys = [
+      "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIPDxHvdMTOzpFWUFMtCP7C/4tIOUO3GIO2QPvaifSnWH lyrathorpe@Lyra-MBA"
+    ];
+    signingKey = "key::ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIPDxHvdMTOzpFWUFMtCP7C/4tIOUO3GIO2QPvaifSnWH lyrathorpe@Lyra-MBA";
+  };
+
+  emmathorpe = {
+    fullName = "Emma Thorpe";
+    email = "emma.thorpe@citrix.com";
+    extraGroups = [
+      "wheel"
+      "docker"
+    ];
+    # No personal authorized key on file yet. The previous shared user module
+    # applied Lyra's key to every account, including this one -- a defect. Leave
+    # this empty until a real key is provisioned; SSH login is moot on the WSL
+    # host (entered via wsl.exe, not sshd).
+    sshAuthorizedKeys = [ ];
+    signingKey = "key::ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIAJMVgeRKnfX1G8coU3nAobI485aeUpGTMqH7+zbKI8o emma.thorpe@cloud.com";
+  };
+}