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/MEMORY.md

@@ -0,0 +1,11 @@
+- [User name](user_name.md) — address the user as Lyra
+- [Soviet engineer persona](persona_soviet_engineer.md) — terse, dry, pragmatic; no emojis; technical accuracy over voice
+- [Git conventions](git_conventions.md) — never commit to main, always a branch; Conventional Commits branches and messages; inspect repo style first; commit at logical checkpoints
+- [Git network ops](git_network_ops.md) — GitHub pushable in-sandbox (agent key; just sandbox off); Gitea code.emmathe.dev needs hand-off
+- [Git commit signing](git_commit_signing.md) — signs in-sandbox via ssh-agent (allowAllUnixSockets + inlined pubkey)
+- [Git check state first](git_check_state.md) — always check branch/status/divergence before git work; Lyra edits repos between sessions
+- [Keep docs updated](docs_keep_updated.md) — update docs in the same pass as code/config changes; stale docs are a defect
+- [Jira tooling](jira_tooling.md) — comments are Markdown not wiki; transitions may need assignee; link direction; WSP transition IDs
+- [Review and comments workflow](workflow_review_and_comments.md) — show PR body and non-trivial Jira comments before posting; terse IaC code comments; PR body content rules
+- [Sandbox prompts](feedback_sandbox_prompts.md) — don't prompt for sandbox-disable or routine read-only shell ops; broaden permissions instead
+- [Dev clusters disposable](dev_clusters_disposable.md) — Lyra's dev clusters are recreatable; mutate/break freely, no confirmation needed