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_network_ops.md

@@ -0,0 +1,18 @@
+---
+name: git-network-ops
+description: Push/pull is remote-specific — GitHub is agent-pushable in-sandbox; Gitea (code.emmathe.dev) needs hand-off to Lyra.
+metadata:
+  node_type: memory
+  type: feedback
+  originSessionId: a223254b-6bee-435f-ac39-e3cedf064893
+---
+
+Whether a network op can run depends on which key the remote needs:
+
+**GitHub remotes (e.g. csg-citrix-storefront/\*): pushable in-sandbox by the agent.** ssh-agent holds the decrypted `~/.ssh/id_ed25519` (`emma.thorpe@cloud.com`), which is authorized on GitHub. Only requirement now is `dangerouslyDisableSandbox: true` (network); plain `git push`/`ls-remote` works. Probe non-mutatively with `git ls-remote` first. (Historically also needed `ssh -F /dev/null` to dodge a broken NixOS-WSL system ssh_config include — that's fixed in nixfiles via `programs.ssh.systemd-ssh-proxy.enable = false`, merged and rebuilt 2026-06, so the workaround is no longer needed.)
+
+**Gitea (`code.emmathe.dev`, e.g. nixfiles): hand off to Lyra.** Needs `~/.ssh/code.emmathe.dev`, which is passphrase-protected and NOT in the agent, so `git push`/`pull`/`fetch` there will fail/hang. Pause, give Lyra the exact command (she runs `ssh-add ~/.ssh/code.emmathe.dev` once, then pushes).
+
+**Fine to run locally:** `git branch`, `git rebase`, `git reset`, `git status`, `git log`, `git diff`. `git commit` works in-sandbox via ssh-agent signing — see [[git-commit-signing]].
+
+**How to apply:** Check the remote host before a network op. GitHub → just do it (sandbox off). Gitea → hand off. Related: [[git-conventions]].