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/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]].