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

users/emmathorpe/work.nix

@@ -0,0 +1,67 @@
+# Work (EDaaS/WSL) home profile: corporate toolchain + tmux tweaks. Git identity
+# comes from the registry (users/registry.nix), not here.
+{ pkgs, lib, ... }:
+
+{
+  # Host-scoped extras for this machine only (the EDaaS/WSL host).
+  imports = [
+    ./renovate-review.nix # daily headless Renovate PR review (systemd user timer)
+  ];
+
+  # The work box keeps its own (corporate) ~/.ssh/config; don't let the personal
+  # programs.ssh (shell.nix) take it over. The ssh-agent below still runs.
+  programs.ssh.enable = lib.mkForce false;
+
+  home.packages = [
+    pkgs.kubectl
+    pkgs.argo-rollouts
+    pkgs.tenv
+    pkgs.kubernetes-helm
+    pkgs.azure-cli
+    pkgs.kubelogin
+    pkgs.curl
+    pkgs.notation
+    pkgs.powershell
+    pkgs.nuget
+    pkgs.gedit
+    pkgs.lens
+    pkgs.python3
+    pkgs.gnumake
+    pkgs.gcc
+    pkgs.libiconv
+    pkgs.autoconf
+    pkgs.automake
+    pkgs.pkg-config
+    pkgs.wget
+    pkgs.google-cloud-sdk
+    # Day-to-day Kubernetes / Helm / Terraform accelerators for this box.
+    pkgs.k9s # cluster TUI
+    pkgs.kubectx # kubectx + kubens (context/namespace switch)
+    pkgs.stern # multi-pod log tail
+    pkgs.dyff # semantic YAML/manifest diffs (Helm release drift)
+    pkgs.tflint # Terraform linter (catches what terraformls won't)
+    pkgs.terraform-docs # generate Terraform module docs
+    pkgs.yq-go # jq for YAML
+  ];
+  services.ssh-agent.enable = true;
+  home.shellAliases = {
+    docker = "/run/current-system/sw/bin/docker";
+  };
+  programs.tmux = {
+    extraConfig = ''
+      			set -g status-right "#(/run/current-system/sw/bin/bash $HOME/code/kube-tmux/kube.tmux 250 red black)"		
+    '';
+  };
+  programs.go = {
+    enable = true;
+  };
+
+  # LSP servers only relevant to work: C# (omnisharp) and Helm charts (helm_ls).
+  # The shared editor (home/editor.nix) carries the universal ones;
+  # these are gated to this host so the heavy omnisharp closure stays off the
+  # personal machines. Tree-sitter grammars (highlighting) remain global there.
+  programs.nixvim.plugins.lsp.servers = {
+    omnisharp.enable = true;
+    helm_ls.enable = true;
+  };
+}