refactor(flake): user registry, multi-user hosts, and portable home outputs (#49)

## Summary

Separates user identity (data) from the reusable Nix modules and lets a host declare any number of users, replacing the previous one-user-per-host structure. Also restructures the tree and exposes the home config for use off these hosts.

## Changes

- **User registry** (`users/registry.nix`): per-user identity (name, email, groups, authorized + signing keys) as the single source of truth; no user data hardcoded in modules.
- **Multi-user `mkHost`**: a host declares a `users` set keyed by username; per-user identity is injected into each home config via the `identity` module arg.
- **Restructured layout**: `users/`, `home/`, `modules/`, `hosts/`, `lib/` replace the former `lyrathorpe/` and `system/` trees.
- **Portable outputs**: standalone `homeConfigurations."<user>@<system>"` (the portable subset — shell, git, editor, claude) plus an exported `homeModules` for use on machines not managed by this flake, or as an input to other flakes.
- Docs (`README.md`, `home/README.md`) and `.gitignore` updated for the new paths.

## Fixes

- Closes #46 — shared user module authorized one user's SSH key for every account.
- Closes #47 — git committer identity hardcoded as defaults instead of per-user.
- Closes #48 — EDaaS systemd linger hardcoded to a literal username.

## Verification

- `nix flake check` passes: treefmt, deadnix, statix, pre-commit, and evaluation of all NixOS hosts + Darwin + homeConfigurations.
- Derivation-path comparison vs `main`: `lyrathorpe-mbp` and `emmathorpe-edaas` are byte-identical; `lyrathorpe-t400`, `lyrathorpe-macpro31` and `lyrathorpe-rpi5` differ only by de-duplicating a repeated `authorized_keys` entry (confirmed with nix-diff — no other change).
- Standalone `homeConfigurations."lyrathorpe@x86_64-linux".activationPackage` builds.

## Notes

- `emmathorpe` has no personal authorized key yet (it previously inherited Lyra's key via the bug in #46); the registry entry is intentionally empty — add a real key if SSH login as `emmathorpe` is wanted (moot on the WSL host).
- A two-repo (public dotfiles / private systems) split is deferred by design; this internal restructure is the prerequisite for it.

---------

Co-authored-by: Emma Thorpe <emma.thorpe@citrix.com>
Reviewed-on: #49

hosts/MBP-Asahi/configuration.nix

@@ -0,0 +1,46 @@
+# MacBook Pro (Apple Silicon, Asahi NixOS). Shared laptop options live in
+# ../../modules/laptop.nix; only host-specific settings are here.
+{ pkgs, ... }:
+
+{
+  imports = [
+    ./hardware-configuration.nix
+  ];
+
+  # UEFI boot via systemd-boot. Asahi manages the EFI vars from macOS, so do not
+  # touch them from NixOS.
+  boot.loader.systemd-boot.enable = true;
+  boot.loader.efi.canTouchEfiVariables = false;
+
+  networking.hostName = "Lyra-Asahi";
+
+  # Audio (PipeWire) and the swaylock PAM stack are inherited from
+  # workstation.nix. hardware.enableRedistributableFirmware is also set there;
+  # it is harmless here since Asahi supplies its own peripheral firmware below.
+
+  # Binary cache for the Asahi kernel/build artifacts, so the MBP pulls prebuilt
+  # outputs instead of compiling the Asahi kernel locally.
+  nix.settings = {
+    substituters = [ "https://nixos-apple-silicon.cachix.org" ];
+    trusted-public-keys = [
+      "nixos-apple-silicon.cachix.org-1:8psDu5SA5dAD7qA0zMy5UT292TxeEPzIz8VVEr2Js20="
+    ];
+  };
+
+  # Apple peripheral firmware (Wi-Fi/Bluetooth). The directory is gitignored and
+  # populated out-of-band -- see README.
+  hardware.asahi.peripheralFirmwareDirectory = ../../modules/firmware;
+
+  environment.systemPackages = with pkgs; [
+    asahi-bless
+    asahi-nvram
+    asahi-btsync
+    asahi-wifisync
+    unzip
+    ppp
+    iptables
+  ];
+
+  # See `man configuration.nix` / the stateVersion docs before changing.
+  system.stateVersion = "25.05";
+}

hosts/MBP-Asahi/hardware-configuration.nix

@@ -0,0 +1,39 @@
+# Do not modify this file!  It was generated by ‘nixos-generate-config’
+# and may be overwritten by future invocations.  Please make changes
+# to /etc/nixos/configuration.nix instead.
+{ config, lib, pkgs, modulesPath, ... }:
+
+{
+  imports =
+    [ (modulesPath + "/installer/scan/not-detected.nix")
+    ];
+
+  boot.initrd.availableKernelModules = [ "uas" "sdhci_pci" ];
+  boot.initrd.kernelModules = [ ];
+  boot.kernelModules = [ ];
+  boot.extraModulePackages = [ ];
+
+  fileSystems."/" =
+    { device = "/dev/disk/by-uuid/217a7427-d799-4e0d-af4e-378db3b27467";
+      fsType = "ext4";
+    };
+
+  boot.initrd.luks.devices."cryptroot".device = "/dev/disk/by-uuid/4ec1a31b-a7ba-41bb-9bbc-4833cfc732b0";
+
+  fileSystems."/boot" =
+    { device = "/dev/disk/by-uuid/420E-1902";
+      fsType = "vfat";
+      options = [ "fmask=0022" "dmask=0022" ];
+    };
+
+  swapDevices = [ {device = "/swapfile"; size = 8192;} ];
+
+  # Enables DHCP on each ethernet and wireless interface. In case of scripted networking
+  # (the default) this is the recommended approach. When using systemd-networkd it's
+  # still possible to use this option, but it's recommended to use it in conjunction
+  # with explicit per-interface declarations with `networking.interfaces.<interface>.useDHCP`.
+  networking.useDHCP = lib.mkDefault true;
+  # networking.interfaces.wlan0.useDHCP = lib.mkDefault true;
+
+  nixpkgs.hostPlatform = lib.mkDefault "aarch64-linux";
+}