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/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.