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/configuration.nix

@@ -0,0 +1,52 @@
+# Apple Mac Pro 3,1 (Early 2008, dual Xeon Harpertown, x86_64). Desktop host:
+# shared graphical/wired options live in ../../modules/desktop.nix; only
+# host-specific settings are here. Install notes (EFI booting, GPU, partitions):
+# see ./README.md.
+{ ... }:
+
+{
+  imports = [
+    ./hardware-configuration.nix
+  ];
+
+  # The Mac Pro 3,1 has 64-bit EFI (confirmed by the owner), so boot via
+  # systemd-boot like the MBP -- no GRUB/BIOS shim needed.
+  boot.loader.systemd-boot.enable = true;
+  # Apple's EFI does not reliably support efibootmgr NVRAM writes; leave the
+  # firmware vars untouched.
+  boot.loader.efi.canTouchEfiVariables = false;
+  # Apple-EFI quirk: if the Mac does not pick up the bootloader at the boot
+  # picker, install it to the fallback path \EFI\BOOT\BOOTX64.EFI and/or
+  # "bless" the ESP from macOS. Uncomment to write the removable fallback path:
+  # boot.loader.efi.efiInstallAsRemovable = true;
+
+  networking.hostName = "MacPro31-NixOS";
+
+  # Elderly host: a compressed RAM swap softens memory pressure (earlyoom in
+  # workstation.nix is the backstop).
+  zramSwap.enable = true;
+
+  # This host accepts SSH, so open 22 (the firewall itself is enabled in
+  # workstation.nix with a default-deny policy).
+  services.openssh.enable = true;
+  networking.firewall.allowedTCPPorts = [ 22 ];
+
+  # Dual Harpertown Xeon microcode. Redistributable firmware (GPU/NIC blobs) is
+  # enabled in workstation.nix.
+  hardware.cpu.intel.updateMicrocode = true;
+
+  # GPU note: the stock card varies between units -- ATI Radeon HD 2600 XT or
+  # NVIDIA GeForce 8800 GT. Sway needs a working KMS/modesetting driver; do NOT
+  # install a proprietary blob here. Depending on the installed card, rely on
+  # the open kernel driver:
+  #   - ATI Radeon HD 2600 XT -> "radeon" (older) or "amdgpu" KMS
+  #   - NVIDIA GeForce 8800 GT -> "nouveau" KMS
+  # These come up automatically via the in-tree drivers + KMS, and the graphics
+  # stack itself is enabled by modules/sway.nix. If a card needs to be forced, add it
+  # here, e.g. `services.xserver.videoDrivers = [ "radeon" ];` (or "nouveau"),
+  # and/or `boot.initrd.kernelModules = [ "radeon" ];` in
+  # hardware-configuration.nix for early KMS.
+
+  # See `man configuration.nix` / the stateVersion docs before changing.
+  system.stateVersion = "26.05";
+}