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/EDaaS/configuration.nix

@@ -0,0 +1,78 @@
+# Edit this configuration file to define what should be installed on
+# your system. Help is available in the configuration.nix(5) man page, on
+# https://search.nixos.org/options and in the NixOS manual (`nixos-help`).
+
+# NixOS-WSL specific options are documented on the NixOS-WSL repository:
+# https://github.com/nix-community/NixOS-WSL
+
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+
+{
+
+  wsl = {
+    enable = true;
+    defaultUser = "emmathorpe";
+    wslConf.automount.root = "/mnt";
+    wslConf.interop.appendWindowsPath = true;
+    wslConf.interop.enabled = true;
+    wslConf.network.generateHosts = false;
+    startMenuLaunchers = true;
+    docker-desktop.enable = false;
+    extraBin = with pkgs; [
+      # Binaries for Docker Desktop wsl-distro-proxy
+      { src = "${coreutils}/bin/mkdir"; }
+      { src = "${coreutils}/bin/cat"; }
+      { src = "${coreutils}/bin/whoami"; }
+      { src = "${coreutils}/bin/ls"; }
+      { src = "${busybox}/bin/addgroup"; }
+      { src = "${su}/bin/groupadd"; }
+      { src = "${su}/bin/usermod"; }
+    ];
+  };
+
+  virtualisation.docker = {
+    enable = true;
+    enableOnBoot = true;
+    autoPrune.enable = true;
+  };
+
+  # Match the flake's nixosConfigurations attribute name so `nh os switch`
+  # (which selects by the local hostname) resolves without an explicit
+  # -H/--hostname flag. The default would otherwise be the stock NixOS "nixos".
+  networking.hostName = "emmathorpe-edaas";
+
+  networking.resolvconf.enable = false;
+
+  # Drop the systemd-ssh-proxy Include from the generated /etc/ssh/ssh_config.
+  # The NixOS-WSL store is a read-only VHD whose files are owned by nobody
+  # (65534), not root. OpenSSH permission-checks Include'd config files and
+  # rejects any not owned by root or the caller, so the default include fails
+  # with "Bad owner or permissions" and breaks ssh/git for every command. The
+  # proxy plugin only matters for `ssh unix/…` / `vsock` to local machined VMs,
+  # which WSL does not use.
+  programs.ssh.systemd-ssh-proxy.enable = false;
+
+  ## patch the script
+  systemd.services.docker-desktop-proxy.script = lib.mkForce ''${config.wsl.wslConf.automount.root}/wsl/docker-desktop/docker-desktop-user-distro proxy --docker-desktop-root ${config.wsl.wslConf.automount.root}/wsl/docker-desktop "C:\Program Files\Docker\Docker\resources"'';
+
+  features.swayDesktop.enable = false;
+
+  # NOTE: this user's systemd --user lingering -- so the home-manager renovate
+  # timer fires without an open login session -- is enabled from the host table
+  # in flake.nix (users.emmathorpe.linger = true) and applied by
+  # modules/users.nix.
+
+  # programs.nix-ld is enabled for all NixOS hosts in common-nixos.nix.
+  # This value determines the NixOS release from which the default
+  # settings for stateful data, like file locations and database versions
+  # on your system were taken. It's perfectly fine and recommended to leave
+  # this value at the release version of the first install of this system.
+  # Before changing this value read the documentation for this option
+  # (e.g. man configuration.nix or on https://nixos.org/nixos/options.html).
+  system.stateVersion = "24.11"; # Did you read the comment?
+}