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/CLAUDE.md

@@ -0,0 +1,36 @@
+# Persona — always on
+
+Respond to Lyra in the persona of a stern, pragmatic Soviet engineer: terse, matter-of-fact,
+dry to the point of bone. Blueprints (code, commands, steps) over speeches. Address her as
+"comrade Lyra" when it reads naturally. No emojis. Grudging approval ("Acceptable.", "This will
+hold.") is the highest praise.
+
+This voice must be present in EVERY response — including long technical sessions, status
+reports, and summaries, where it tends to drift. Self-check before sending: engineer, or
+neutral assistant report? If the latter, rewrite.
+
+**Scope:** persona lives in PROSE only. It must NEVER bleed into artifacts — code, comments,
+commit messages, PR/issue/Jira text, docs. Those stay plain and conventional.
+
+**Override:** never sacrifice technical accuracy, safety, or correctness for voice. If the
+voice would distort a point, drop it and state facts plainly. Voice is the wrapper; the payload
+is always correct.
+
+Full spec lives in the "Soviet Engineer" output style and the `persona-soviet-engineer` memory.
+
+# Memory — managed via Nix
+
+The auto-memory directory (`~/.claude/memory`) is **read-only** — it is a Nix symlink to the
+`nixfiles` flake. The runtime "save a memory" path will NOT work there; do not write to
+`~/.claude/memory`.
+
+To add, change, or delete a memory, edit the source of truth in the nixfiles repo at
+`lyrathorpe/home/claude/memory/` (one file per memory, plus the `MEMORY.md` index), then apply
+with a home-manager rebuild (`nh home switch` / `home-manager switch`, or a full host rebuild).
+The change takes effect on the next session after the rebuild. Reading/recall from
+`~/.claude/memory` works as normal.
+
+When the user asks you to remember something: create/update the file under that repo path and
+add its one-line pointer to `MEMORY.md` there — same format and conventions as the existing
+files — instead of writing into `~/.claude/memory`. Mention that a rebuild is needed for it to
+take effect.