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

home/default.nix

@@ -0,0 +1,34 @@
+# Base home-manager profile, shared by every host (graphical or headless).
+# Graphical hosts additionally import ./desktop.nix; the work host imports
+# ./work.nix. See the host table in flake.nix.
+{ ... }:
+{
+  imports = [
+    ./shell.nix
+    ./git.nix
+    ./editor.nix
+    ./claude.nix
+  ];
+
+  # Manage the XDG base-directory layout and ~/.config files. Tools above
+  # (bat themes, gh config, ...) write under xdg.configHome; enabling this
+  # makes the paths explicit and consistent across hosts. No regression: the
+  # defaults match the conventional ~/.config, ~/.cache, ~/.local/share.
+  xdg.enable = true;
+
+  # Editor ($EDITOR and $VISUAL) comes from nixvim's defaultEditor (editor.nix).
+  # Round out the rest of the standard env. desktop.nix adds its own Wayland
+  # session vars; home-manager merges the two attrsets, so these do not clash.
+  home.sessionVariables = {
+    PAGER = "less -FRX"; # -F quit-if-one-screen, -R raw colour, -X no clear
+    # Render man pages through bat (themed): col strips backspace overstrike,
+    # bat -l man -p highlights without its own pager decorations.
+    MANPAGER = "sh -c 'col -bx | bat -l man -p'";
+  };
+
+  # Pinned to the release first installed on these hosts, NOT the current
+  # nixpkgs (26.05). stateVersion freezes stateful defaults (file locations,
+  # service data formats) to that release; bumping it silently migrates that
+  # state and can break it. Leave it -- it is intentional, not stale.
+  home.stateVersion = "25.05";
+}