refactor(flake): add user registry and multi-user host support

Separate user identity (data) from the reusable modules, and let a host
declare any number of users instead of exactly one.

- users/registry.nix: per-user identity (name, email, groups, authorized
  and signing keys) as the single source of identity; no user data is
  hardcoded in the modules.
- mkHost takes a `users` set keyed by username; per-user identity is
  injected into each home config via the `identity` module arg
  (extraSpecialArgs is per-host, so it cannot carry per-user data).
- modules/users.nix builds accounts from the registry; modules/ssh.nix no
  longer defines authorized keys (the registry owns them); home/git.nix
  and home/desktop.nix read `identity`; users/emmathorpe/work.nix drops
  its now-redundant git identity override.
- Restructure the tree: users/, home/, modules/, hosts/, lib/ replace the
  former lyrathorpe/ and system/ layout.
- Add standalone homeConfigurations (the portable subset: shell, git,
  editor, claude) and an exported homeModules output for use on machines
  not managed by this flake, or as an input to other flakes.

Behaviour-preserving for existing hosts: lyrathorpe-mbp and
emmathorpe-edaas evaluate to identical derivations; lyrathorpe-t400,
lyrathorpe-macpro31 and lyrathorpe-rpi5 differ only by de-duplicating a
repeated authorized_keys entry. Fixes the SSH authorized-key leak (one
user's key was applied to every account), the hardcoded default git
identity, and the hardcoded EDaaS linger setting.

hosts/T400/README.md

@@ -0,0 +1,48 @@
+# ThinkPad T400 — install notes
+
+Flake host: `lyrathorpe-t400`. Files: `configuration.nix`, the `boot-*.nix`
+variants, and `hardware-configuration.nix`.
+
+## Hardware configuration
+
+`hardware-configuration.nix` here is a hand-written **placeholder**. On the real
+machine, run `nixos-generate-config`, replace the file, and commit it. It assumes
+by-label partitions — root `nixos` (ext4) and `swap` — so either label them at
+install time or swap in the generated UUIDs.
+
+## Bootloader — import the module matching the flashed firmware
+
+`configuration.nix` imports exactly one boot module. Default is `boot-bios.nix`;
+switch by commenting it out and uncommenting the relevant alternative.
+
+| Firmware                                             | Module                   | Notes                                                                                                                                                                                                                                                                                 |
+| ---------------------------------------------------- | ------------------------ | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
+| Stock Lenovo BIOS, or coreboot + **SeaBIOS** payload | `boot-bios.nix`          | GRUB on the MBR. Set `device` to the real install disk (`/dev/sda` by default). MBR/legacy layout.                                                                                                                                                                                    |
+| coreboot + **GRUB** payload                          | `boot-coreboot-grub.nix` | GRUB is config-only (`device = "nodev"`); NixOS does **not** write to a disk. Your coreboot `grub.cfg` (in the flash chip) must `search` for and `configfile` the on-disk `/boot/grub/grub.cfg`, or chainload the disk's GRUB.                                                        |
+| coreboot + **Tianocore/edk2 (UEFI)** payload         | `boot-coreboot-uefi.nix` | systemd-boot. `canTouchEfiVariables = true` (coreboot honours NVRAM writes). The module **declares its own ESP** (`/boot` vfat, label `ESP`) — when you regenerate `hardware-configuration.nix`, do **not** let it also define `/boot`. Create + label an `ESP` vfat partition (GPT). |
+
+## Graphics
+
+This unit has the optional **discrete ATI Mobility Radeon HD 3470 (RV620)**. The
+open `radeon` KMS driver is loaded in the initrd for early modesetting; firmware
+comes from `enableRedistributableFirmware`.
+
+The T400 has switchable graphics (discrete ATI + Intel GMA 4500MHD). Select
+**Discrete** in the firmware's graphics setting so only the ATI is live. If you
+run **Integrated** instead, the Intel `i915` driver takes over with no config
+change and `radeon` stays idle.
+
+## 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 radeon/i915 KMS (see Graphics).
+
+## Apply
+
+```sh
+sudo nixos-rebuild switch --flake .#lyrathorpe-t400
+```