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/claude.nix

@@ -0,0 +1,33 @@
+# Claude Code, configured declaratively via home-manager. Wanted on every host.
+#
+# The STATIC config is managed here: the global CLAUDE.md (persona/context), the
+# custom output style, and the auto-memory directory. settings.json is
+# deliberately left UNMANAGED -- Claude Code rewrites it at runtime (interactive
+# permission grants, /config), and a read-only /nix/store symlink would break
+# those writes.
+#
+# Memory is the source of truth in this repo (./claude/memory). It is symlinked
+# read-only into ~/.claude/memory, so the runtime "save a memory" path no longer
+# writes there -- recall still works, but new/changed memories must be added to
+# this repo and rebuilt. CLAUDE.md instructs Claude to do exactly that.
+{ ... }:
+{
+  programs.claude-code = {
+    enable = true;
+    # package defaults to pkgs.claude-code (tracked to unstable via the flake
+    # overlay); installs the CLI on every host.
+
+    # ~/.claude/CLAUDE.md -- global instructions / persona / memory workflow.
+    context = ./claude/CLAUDE.md;
+  };
+
+  home.file = {
+    # Custom output style. The module has no option for output-styles/, so place
+    # it directly; selection (settings.json `outputStyle`) stays mutable.
+    ".claude/output-styles/soviet-engineer.md".source = ./claude/output-styles/soviet-engineer.md;
+
+    # Auto-memory directory, Nix-managed (read-only). Edit ./claude/memory in
+    # this repo and rebuild to change what Claude remembers.
+    ".claude/memory".source = ./claude/memory;
+  };
+}