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/MacPro31/README.md

@@ -0,0 +1,61 @@
+# Mac Pro 3,1 (Early 2008) — install notes
+
+Flake host: `lyrathorpe-macpro31`. Desktop (`portable = false`, imports
+`../../modules/desktop.nix`). Files: `configuration.nix`,
+`hardware-configuration.nix`.
+
+## Hardware configuration
+
+`hardware-configuration.nix` here is the real config generated by
+`nixos-generate-config` on the machine. Root is an **LVM** logical volume
+(`/dev/mapper/MacPro-Root`, ext4); the ESP (vfat) and swap are referenced by
+UUID. The initrd carries `dm-snapshot` for the LVM root. Regenerate and commit
+if the disk layout changes.
+
+## Bootloader
+
+The Mac Pro 3,1 has **64-bit EFI**, so it uses **systemd-boot** (no GRUB/CSM
+shim). `canTouchEfiVariables = false` because Apple's firmware does not reliably
+accept `efibootmgr` NVRAM writes.
+
+Apple-EFI quirk: if the firmware boot picker does not show NixOS after install,
+either
+
+- uncomment `boot.loader.efi.efiInstallAsRemovable = true;` in
+  `configuration.nix` (installs the fallback `\EFI\BOOT\BOOTX64.EFI`), and/or
+- "bless" the ESP from macOS.
+
+Partition the disk GPT with an ESP (vfat).
+
+## Graphics
+
+The stock card varies between units — **ATI Radeon HD 2600 XT** or **NVIDIA
+GeForce 8800 GT**. No proprietary driver is hardcoded; Sway relies on in-tree KMS:
+
+- ATI Radeon HD 2600 XT → `radeon` (or `amdgpu`) KMS
+- NVIDIA GeForce 8800 GT → `nouveau` KMS
+
+These come up automatically. If a card needs forcing, set
+`services.xserver.videoDrivers` and/or add the module to
+`boot.initrd.kernelModules` for early KMS (see the comment in
+`configuration.nix`).
+
+## Networking
+
+Wired Ethernet via NetworkManager (from `desktop.nix`) — the Mac Pro has two
+gigabit ports.
+
+## Login
+
+Graphical login via a Wayland greeter — `greetd` running ReGreet inside the
+`cage` kiosk compositor — configured centrally in `lyrathorpe/swaywm.nix` for
+every Sway host (gated on `features.swayDesktop.enable`). The greeter is forced
+to the Dvorak layout to match the console and Sway session. Set the user
+password (`passwd lyrathorpe`) after install, or the greeter cannot
+authenticate. Requires working KMS (radeon/nouveau — see Graphics).
+
+## Apply
+
+```sh
+sudo nixos-rebuild switch --flake .#lyrathorpe-macpro31
+```