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/workstation.nix

@@ -0,0 +1,49 @@
+# Form-factor-agnostic base for the physical graphical NixOS machines. Imported
+# by both ./laptop.nix and ./desktop.nix; those add only the bits that differ
+# between portable and desktop hosts (chiefly the networking backend).
+#
+# The bootloader is NOT set here -- it is firmware-specific, not form-factor:
+# UEFI hosts (MBP, Mac Pro 3,1) use systemd-boot, the BIOS-only T400 uses GRUB.
+# Each machine config declares its own.
+{ lib, pkgs, ... }:
+{
+  features.swayDesktop.enable = true;
+
+  console.keyMap = "dvorak";
+
+  # Intel thermal management. x86 only -- the Asahi MBP governs its own SoC
+  # thermals, and thermald is an Intel-platform daemon.
+  services.thermald.enable = lib.mkIf pkgs.stdenv.hostPlatform.isx86_64 true;
+
+  # Default-deny inbound. Hosts that run a listening service open their own
+  # ports next to where the service is enabled (e.g. sshd -> 22 on X1).
+  networking.firewall.enable = true;
+
+  # Disk hygiene for the physical hosts. fstrim reclaims unused SSD blocks on a
+  # weekly timer; cleanOnBoot wipes /tmp at every boot.
+  services.fstrim.enable = true;
+  boot.tmp.cleanOnBoot = true;
+
+  # Userspace OOM killer: act on memory pressure early instead of letting the
+  # kernel OOM-thrash. Matters on the 4 GiB T400 and the elderly Mac Pro.
+  services.earlyoom.enable = true;
+
+  # Firmware updates via LVFS. No-op on the Asahi MBP (Apple-managed firmware),
+  # useful for UEFI/SSD updates on the x86 hosts.
+  services.fwupd.enable = true;
+
+  # Audio. PipeWire with the PulseAudio shim covers every graphical host; no
+  # per-machine audio config is needed.
+  services.pipewire = {
+    enable = true;
+    pulse.enable = true;
+  };
+
+  # swaylock PAM stack. None of these machines has working fingerprint auth, so
+  # an empty service is enough -- swaylock falls back to password.
+  security.pam.services.swaylock = { };
+
+  # Redistributable firmware (GPU/Wi-Fi/NIC blobs) for the x86 hosts. Harmless
+  # on the Asahi MBP, which supplies its own peripheral firmware out-of-band.
+  hardware.enableRedistributableFirmware = true;
+}