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/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]].