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/docs_keep_updated.md

@@ -0,0 +1,14 @@
+---
+name: docs_keep_updated
+description: "Keep documentation in sync with every change as part of the work, not a separate step"
+metadata:
+  node_type: memory
+  type: feedback
+  originSessionId: ca09fbe4-9226-4ad9-874f-04df90840eef
+---
+
+When changing config or code, update the affected documentation in the same pass — READMEs, KEYBINDINGS, per-host install notes, module comments. Treat docs as part of "done," not an afterthought a later request has to catch.
+
+**Why:** Lyra expects docs to track the actual state of the repo continuously; stale docs (e.g. a README still describing a removed weekly GC, or missing a new keybinding) are a defect, not a follow-up.
+
+**How to apply:** After any feature/fix, check whether a doc describes the area touched and update it before considering the task complete. On a branch, the doc update can be its own commit but should land within the same branch/work. Relates to [[git_conventions]] and [[workflow_review_and_comments]].