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

modules/common-nixos.nix

@@ -0,0 +1,62 @@
+# Options shared by every NixOS host (laptops and the WSL box). Imported via
+# baseModules in flake.nix. Host- and platform-specific settings stay in the
+# per-machine configs; laptop-only settings live in ./laptop.nix.
+{ pkgs, ... }:
+{
+  time.timeZone = "Europe/London";
+  i18n.defaultLocale = "en_GB.UTF-8";
+
+  # Store hygiene. auto-optimise-store hard-links identical files in the store
+  # after each build (cheap dedupe; NOT a garbage collector -- there is
+  # deliberately no automatic GC timer). The larger download buffer avoids
+  # "buffer full" stalls when fetching big NARs over a fast link.
+  nix.settings.auto-optimise-store = true;
+  nix.settings.download-buffer-size = 134217728; # 128 MiB
+
+  # Extra binary cache for the nix-community toolchain (home-manager, nixvim,
+  # treefmt, ...). Merges with any host-specific caches (e.g. the Asahi cache on
+  # the MBP) rather than replacing them.
+  nix.settings.substituters = [ "https://nix-community.cachix.org" ];
+  nix.settings.trusted-public-keys = [
+    "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs="
+  ];
+
+  # Run dynamically-linked foreign binaries (VS Code remote server, prebuilt
+  # toolchains, language-server downloads) on every NixOS host, not just WSL.
+  programs.nix-ld.enable = true;
+
+  # Minimal system-level CLI available before the home-manager profile loads
+  # (e.g. early boot / rescue). User-level tooling lives in home-manager.
+  environment.systemPackages = with pkgs; [
+    git
+    fastfetch
+  ];
+
+  # Fonts on every host. The Nerd Font carries the powerline/Nerd glyphs the
+  # tmux statusline uses (foot names it explicitly in home/sway.nix); Noto sans +
+  # colour emoji prevent tofu in terminals/TUIs/Firefox -- important on the WSL
+  # box, which does not pull the graphical hosts' default Noto stack. The Mac
+  # installs the Nerd Font via the Darwin config.
+  fonts.packages = with pkgs; [
+    nerd-fonts.jetbrains-mono
+    noto-fonts
+    noto-fonts-color-emoji
+  ];
+  # Map the generic fontconfig families so anything asking for "monospace" gets
+  # the Nerd Font (with emoji fallback), not DejaVu.
+  fonts.fontconfig.defaultFonts = {
+    monospace = [
+      "JetBrainsMono Nerd Font"
+      "Noto Color Emoji"
+    ];
+    sansSerif = [
+      "Noto Sans"
+      "Noto Color Emoji"
+    ];
+    serif = [
+      "Noto Serif"
+      "Noto Color Emoji"
+    ];
+    emoji = [ "Noto Color Emoji" ];
+  };
+}