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.

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;
+  };
+}