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_commit_signing.md

@@ -0,0 +1,22 @@
+---
+name: git-commit-signing
+description: "Commits sign in-sandbox via ssh-agent — needs `allowAllUnixSockets: true` in settings, plus pubkey inlined in user.signingkey."
+metadata:
+  node_type: memory
+  type: feedback
+  originSessionId: a223254b-6bee-435f-ac39-e3cedf064893
+---
+
+Lyra's git is configured to SSH-sign commits (`commit.gpgsign=true`, `gpg.format=ssh`). The sandbox masks `~/.ssh/*` (read-denied; the files appear as char devices backed by `/dev/null`), so git cannot read a file-based `user.signingkey` and ssh-keygen cannot read the private key directly. Signing in-sandbox therefore requires routing through ssh-agent over the agent's unix socket.
+
+**Working setup (as of 2026-06-02):**
+
+1. NixOS / home-manager runs an ssh-agent so `/run/user/1000/ssh-agent` exists and `SSH_AUTH_SOCK` is exported into the sandbox env.
+2. `~/.claude/settings.json` has `sandbox.network.allowAllUnixSockets: true` to let the sandbox `connect()` to that socket. On Linux/WSL2 this is the ONLY available switch — the per-path `sandbox.network.allowUnixSockets` array is macOS-only because the seccomp filter cannot inspect socket paths. Tradeoff: every unix socket on the host (including `/var/run/docker.sock` if present, DBus, etc.) becomes reachable from sandboxed commands.
+3. `user.signingkey` set to the inlined pubkey: `git config --global user.signingkey "key::$(cat ~/.ssh/id_ed25519.pub)"`. Must run with DOUBLE quotes outside the sandbox so `$(...)` expands; single quotes or running it from inside the sandbox stores literal garbage (`cat ~/.ssh/id_ed25519.pub` reads `/dev/null` in-sandbox).
+
+**Why:** removes the per-commit `! git commit ...` friction; private key stays in the agent, never enters the sandbox.
+
+**How to apply:** Commit normally with `git commit`. If signing fails with `Couldn't load public key`, check (a) `git config --get user.signingkey` starts with `key::ssh-ed25519 AAAA...` (not literal `$(...)`), (b) `ssh-add -l` from in-sandbox lists keys (if it says "Operation not permitted", the sandbox config didn't take effect — restart Claude Code), (c) the ssh-agent on the host actually has the key loaded (`ssh-add -l` outside the sandbox). Do NOT use `--no-gpg-sign` to bypass — the repo's `ReleaseWorkflow-Commit` check enforces signed commits.
+
+Related: [[git-network-ops]], [[git-conventions]].