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/features.nix

@@ -0,0 +1,12 @@
+# Feature-flag option declarations shared by every NixOS host (imported via
+# baseModules in flake.nix). Declaring the flags here -- rather than inside the
+# module that implements them -- means a host can read or set a flag without
+# importing the (often large) implementation module. In particular,
+# features.swayDesktop.enable is read by modules/users.nix on every host, but a
+# headless host (e.g. the Pi) must be able to leave it at its default without
+# pulling in modules/sway.nix. The implementation lives in modules/sway.nix,
+# gated on this flag.
+{ lib, ... }:
+{
+  options.features.swayDesktop.enable = lib.mkEnableOption "the Sway desktop";
+}