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/memory/git_check_state.md

@@ -0,0 +1,14 @@
+---
+name: git_check_state
+description: "Always check real git state (branch, ahead/behind, log) before git work — Lyra edits repos between sessions"
+metadata:
+  node_type: memory
+  type: feedback
+  originSessionId: ca09fbe4-9226-4ad9-874f-04df90840eef
+---
+
+Before starting any git-related work — and again before committing, amending, or resetting — inspect the actual repo state: current branch, `git status -sb` (ahead/behind), and the recent log including `origin/<branch>..` and `..origin/<branch>`. Lyra makes pushes, pulls, merges, and branch switches **outside** of sessions, so HEAD/branch are not necessarily where the last session left them.
+
+**Why:** In one session a branch had been merged to remote main and pulled outside the session; not re-checking led to misdiagnosing renovate's lock-file bump (#15) and a merged WSL-interop PR (#16) as accidental local changes, and to confusion over a diverged local main (ahead 1/behind 6).
+
+**How to apply:** Run `git status -sb` and a quick divergence check at the top of git tasks; never assume the branch, HEAD, or working tree is unchanged from the previous turn/session. Reconcile against `origin/<branch>` before building on top. Relates to [[git_conventions]] and [[git_network_ops]].