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/docker.nix

@@ -0,0 +1,34 @@
+# Docker host with the daemon socket exposed over the network.
+#
+# SECURITY: the daemon listens on plain TCP 2375 with NO TLS and NO auth. Access
+# to that port is root-equivalent on this host (the Docker API can mount the
+# host filesystem and run privileged containers). The ONLY thing protecting it
+# is the nftables rule below, which accepts 2375 solely from the trusted LAN
+# subnet. Do not widen that subnet to anything you do not fully trust. The
+# secure upgrade path is mutual TLS on 2376 (--tlsverify with client certs);
+# that needs out-of-band cert provisioning and is intentionally not wired here.
+{ ... }:
+{
+  virtualisation.docker.enable = true;
+
+  # Expose the daemon over TCP by extending systemd socket activation rather than
+  # setting daemon.settings.hosts. The NixOS docker unit starts dockerd with
+  # `-H fd://` and takes its listeners from this socket; putting `hosts` in
+  # daemon.json as well would conflict with that and dockerd would refuse to
+  # start. Adding the TCP listener here keeps a single source of truth.
+  # The leading "" resets the unit's default (unix-socket-only) ListenStream list.
+  systemd.sockets.docker.socketConfig.ListenStream = [
+    ""
+    "/run/docker.sock"
+    "0.0.0.0:2375"
+  ];
+
+  # Source-restricted firewall rule for the Docker TCP port. 2375 is deliberately
+  # NOT added to networking.firewall.allowedTCPPorts (that would open it to every
+  # source); instead nftables accepts it only from the trusted subnet. Adjust the
+  # CIDR to match the LAN that should reach the Docker API.
+  networking.nftables.enable = true;
+  networking.firewall.extraInputRules = ''
+    ip saddr 10.187.1.0/24 tcp dport 2375 accept
+  '';
+}